The first thing you need to do is figure out: Why Are The Players Doing That?
Answer A: Because They Like That Archetype
I've known players who always want to play something that hits things with a sword, and that's it. I've known players who always want to play healers. I've known players who always want to have some kind of pet. etc.
Those people are playing that because they really like that archetype, and they're happy playing it. There's really nothing "wrong" with that, and so changing the situation in a way that makes people happy requires a delicate hand.
Since you're not interested in removing player options, the only thing you can do with these people is figure out what it is that they really like about what they're playing, and show them how they can get it in another archetype. Sometimes that's easy ("I want to have a pet" can actually be done a lot of different ways, so you can mix it up). Sometimes what they want to play is so specific that there may not be much you can do unless you're willing to create a custom class for them.
At the end of the day, it's not really a big problem if they're having fun playing the archetype in question.
Answer B: Because It Works
I've also known players who play something because they believe it's going to be the most useful thing in the campaign. This can be particularly true in a game like D&D, where it's usually assumed that combat is going to happen but it can't be assumed that diplomacy/sneaking/other things are going to happen. In the case of these people, they're not opposed to playing something else so long as it's going to be effective on a regular basis.
For these people, the answer is easier:
- Tell them that other things will be needed/useful. If you want someone to play a "party face" social type character, make sure they know the campaign is structured that a character like that will be effective.
- Actually give those roles a chance to be effective.
Number 2 is where things can often go wrong. If everyone favors combat focused archetypes in your game, maybe it's because you usually run games that are combat focused. Players who want to be effective are going to want to focus on what you're throwing at them, and if every problem can be solved by hitting it hard enough, that's what they'll focus on.
I know someone who when he GMs, places a huge focus on the world building and social aspect of his game. When I'm playing in his game, I know I can make a character that's a lousy combatant but great at something else, and I will not feel like I suck all the time. That's huge, and gives me confidence to make a character like that. In other games I would never deliberately make a character that's bad at combat, because I'll just wind up not doing anything most sessions (or doing it poorly).
Actually, my next character in his new campaign is going to be a barmaid. In a party full of heroes. The only reason that works is because I know there will be times when being a barmaid is just as useful as being a death dealing Wizard.