I'm a pretty new DM since normal DM wanted to play for once, the party is 8th level and was recently faced with a challenge that their info says they can't handle. Since they are near a major city one of the players thought to go enlist the help of higher level characters (15-20th level) I can't seem to think of a logical reason why they wouldn't or why anyone wouldn't help them. Why can PCs not simply call on high level NPCs to address major threats?
Higher level NPCs have their own problems. They have a Kingdom to run, an important line of research to finish, the Grxlits of Grbasdn are about to consume the realm of SJDJJ and throw 10 million inhabitants into the fires of eternal stench, etc.
Imagine that next time your 8th level party is in hot pursuit of an evil cultist who's trying to infect the realm with a weeping plague and a bunch of level 1 adventurers come up and ask them to spend the next 2 weeks hunting down a group of basic goblins. What are they going to say? "not worth our time, you deal with it". Firstly the xp/loot reward is insignificant, and secondly the threat is just not as important as the one they are already dealing with.
The NPCs may offer some aid that they can spare, maybe some scrolls or potions or advice. They may even if their goals are suitable aligned offer more significant help or even send a minion along but they have far more important things to handle than getting involved in a trifling matter like this personally.
Higher level NPCs are likely to have their own lives and agendas; unless they're mercenaries, they probably make a living doing something other than waiting for people to pay them to risk their lives in a fight that isn't theirs (and if they are mercenaries, it's pretty likely they're less powerful than PCs, else whoever got the PCs into the adventure would have sought out the powerful mercenaries instead).
Also, as noted in comments, if there were available, more powerful NPCs, they'd likely have demanded more (more loot, more payment, more intangibles like political power or trade agreements) than the originators were willing to give -- which implies more than the PCs are able to pay.
It would be as if a police officer decided to call the president or the head of the FBI to enlist them for catching a burglar. Surely, since they are higher level, they should resolve the investigation in no time, right?
Heck, forget about getting help: just think about how hard it will be to merely meet them! Why would they care about your low level issues? Put yourself in the shoes of a Level 15 Wizard. What do these lowly adventurers have that could convince you to help them? Even if you could solve the whole situation by casting a single spell, soon there will be lines of people outside your tower wanting your help too. Nah, you already went through this when you were low level yourself and you have more interesting things to do now.
If high level NPCs are feeling generous, they may give some advice, words of encouragement and some kind of minor help, like a potion or use a low-level spell for them. But it would be absurd to think they have spare time to waste on small stuff.
This is a part of worldbuilding that a lot of people get bitten by: why is this specific group the one that has to deal with threats? Why not someone more competent?
If your adventuring party is treasure-hunting, this is not so hard. But if they're dealing with a potential threat to the city, they might reasonably expect the city's NPCs to help them out.
The only solution I've found that works is to declare that there aren't any higher-level NPCs in the city. This is similar to the ideas of E6, although you don't have to go quite that extreme: you simply declare that NPCs don't advance past sixth level. (With perhaps the exception of a few city leaders who can advance to seventh level if the party really needs fourth level spells.) That way the player characters actually are the heroes and the defenders of the city.
Note that this does introduce wrinkles if the party needs high-level magic such as raise dead. Your options for that are to hand-wave it (somebody can cast the spell off a scroll, or there's a ritual to do it, or something of that nature) or to simply do without.
You’re a lucky DM
Your players are giving you an opportunity to develop the shared storytelling of your campaign. They wish to engage with your world — that makes for much better play than PC’s that just seek out the next monster to kill and loot.
Since you feel this would be reasonable in your game world, let them find the help they seek. You just need to figure out what the cost is.
Help doesn’t mean solving the problem for the party
Simply put, if the party seeks out aid, it doesn’t harm the game if they find it, as long as the PC’s still have an important role to play, the challenge is still appropriate, and everyone is having fun.
No free lunch
Even the most honorable paladin might expect something from the party in return. The party might need to:
- Make a donation to the church or other organization
- Agree to take on a quest
- Join/swear loyalty to an organization
Less upright organizations might require other compensation, perhaps a fair (or out-sized) share of any treasure found. The possibilities are endless. Whatever the cost is, make sure it helps you tell your story.
Deduct the appropriate XP’s
If you are tracking experience points, reduce the amount given to the PC’s for challenges they meet with help. Most players crave XP’s, so this will be an incentive for them to solve problems with as little outside help as possible.
Beware of Sidelining
(Sidelining is a GM’ing no-no where PC’s become ancillary to the action occurring among more powerful NPC’s — this is generally not fun for the players for obvious reasons.)
It’s a different case when the PC’s are actively seeking help. It can be fun enlisting the aid of a metallic dragon and following it into battle. Eventually though, if the dragon is getting all the glory, this will get old.
To avoid sidelining, the help may also come in the form of “buffing” spells and single/limited use items, a squad of low-level soldiers to command, or even just valuable information.
Patron NPC’s should interesting to the players
It’s quite possible that any patron or patrons the party deals with may become a long term presence in your campaign. Come up with a few NPC character concepts that you find interesting, and see which ones the players seem interested in.
If the party enjoys working with the patron, you will have an “easy lever” to nudge the party towards any course of action you want. Alternatively, the relationship can become adversarial, sooner or later.