In reality, the necklace of fireballs simply isn’t that good
Sure, it’s easier to use than a scroll or wand of fireball—though that tends not to matter very much to an adventuring party, since much of the party is going to be able to use wands, either as a result of their own spellcasting or because the UMD check is pretty easy to hit. Having low-level, untrained NPCs along with you to hand out the necklace to is just a logistical mess and unlikely to be effective, even if you can get past the issue whereby the followers provided by Leadership are not suicidal. Maybe a thrallherd could do it, but, uh. A thrallherd with a legion of suicide bombers is not a character I’d be comfortable playing with.
And sure, you can horde a whole bunch of them together, if you really want to, for one massive blast—expensive, though, and an eternal wand of explosive runes can do the same thing more cheaply. Harder to detonate, but usually that is a good thing when dealing with explosives—Alfred Nobel made the fortune the funds the Nobel Prizes because he invented TNT to replace nitroglycerin, not because TNT provided more force but because nitroglycerin explodes if you look at it too hard, while TNT you actually need to be trying to get it to explode. In the world of explosives chemistry, making new chemicals that blow up is easy—it’s making new chemicals that blow up only when you want them to that’s fiendishly difficult.
And all of these things are in comparison to a wand of fireball or an eternal wand of explosive runes—which are not good items themselves. Direct-damage spells tend to be rather difficult to use, requiring particular feats and class features to improve them because the base spells are on the weaker side. Otherwise, weapon attacks are usually better, and obviously those don’t use spell slots. The spells certainly can be used, of course, but when you put them in an item, you eliminate almost all of the options for improving them—even just increasing their caster level costs lots and lots of gold.
So in practice, all three of these items are pretty poor, and the necklace of fireballs is for most situations in most parties the worst of the three. You are almost-always better off just selling each of these and buying something more useful to you.
D&D 3.5e isn’t balanced and not everything has a real benefit
There are many, many, many options in this game that are strictly inferior to other options. Necklace of fireballs is a nice example, but there are more serious ones. I mean, consider the class tiers—this is not a balanced game. Any time you find an item or a feat or whatever else that just seems straight-up weaker than something similar, chances are it is simply weaker. It happens all the time. No harm in asking, of course, but it’s something to bear in mind.
Which means the necklace of fireballs is only really useful as a trap
If it’s in a pile of loot that is near where combat might happen, it might explode. If it is in a pile of loot and the players take it with them as they continue through the dungeon and get in more combats, it might explode again. It’s a trap—or, if you want to be generous, a gamble the players could take, can we keep this thing intact long enough to sell it, or is it going to blow up in our faces? But it seems to see most of its use as a kind of juvenile “gotcha!” gimmick for DMs who find that kind of thing “fun.”