It's just a vanity thing, pure and simple.
I have been on this forum since the beginning and I have seen dozens upon dozens of threads asking Bioware to make >insert race, color, body type, sexual orientation< inclusive so they can feel represented. Heck, there are already countless threads for ME4 asking those things. I know I am never going to be represented in a game for who I am and that's okay. I don't mind. I don't really want a deaf character in a video game, I play one in real life and it's not as great as one would think. Bioware doesn't have to make a protagonist that is 5 ft 1 and has the body shape of a fourteen year old. Again, that is fine. I don't mind. I can go play The Last of Us with Ellie if I want that (her body type is the only video game character that is even close to my size). But at least give me the chance to not have to have a hulking figure. I have seen so many threads saying that, "femShep's arms are too skinny for her to be a solider. She is too svelte to be taken seriously as a soldier. No one is going to believe she is a badass. She needs more muscle!" and so forth and so on. But I like playing a small framed woman who is still able to kick **** and be a badass while doing it.
So, yeah. I am not going to lie and say I want light armor to move more stealthy, or to be able to move faster or any other combative reason. Though, I do believe Mass Effect has shown light armor can be effective and that there is a place for it. I want sleeker armor because it makes me feel inclusive and aesthetically I think it looks better. So, pure vanity reasons and I am not ashamed to admit it. It is a game, after all. Everyone wants to relate in some way, this is just my way.
And really, a lot of people (not all) just want heavy armor because they believe it looks stronger, more soldier like, etc. And that is fine. I hope they always have that option, but it is kind of self-serving to want something that appeals to you, but deny that to anyone else. When it comes down to actually playing the game, stats, upgrades and perks and skills are what really keeps one alive and doing well. It only has to be in the realm of possibility to be believable and since light armor is already established, what harm would it be to allow someone else to have the options they want, as well as your own? Nothing. It wouldn't hurt you at all.
Disclaimer: Any "you" is used in general context and not anyone specific.