This is a response I wrote in this thread, and it's also something that's been bugging for a while. I thought it might be a food for thought for you guys.
I think the limited customization of mediums and lights is a major contributing factor to this. For example, what are the weapons you can mount with decent speed, armor and ammo on IS lights?
Flamer, SL, ML, LL, ERLL
Missile: SSRM2, SRMs, LRM5
LL and ERLL builds are usually terrible when not paired with ECM and heavier AC builds are prohibitely heavy. AC2 is in a such state that there's no point in bringing it at all. SL, supposedly "the" weapon of lights, is utter garbage. This leaves lights with just MLs, MGs and Missiles. The differentiating factors of lights just remain as the chassises and their individual quirks/hitboxes. You can only play the same ML, MG and SRM builds so much before you get bored. I crossed SL and AC2 because they're just terrible weapons, Flamer terrible.
How about mediums?
Flamer, SL, ML, LL, ERLL, PPC
Missile: SSRM2, SRMs, LRM5-10
AC2, AC5, UAC5, AC10, LBX10
With the exception of the Shadowhawk, pretty much all you can do is combinations of MLs, SRMs and LRMs. Most of the LL builds are too hot and heavier AC10-LBX10 builds heavily cripple the heat efficiency and speed. AC20/Gauss 'Mechs are possible, but they're so specialized that most of the time they're not effective. Mediums also don't handle the heat of PPCs well, and their superiority is long gone with the speed nerf. The customization of medium 'Mechs is also limited, but still a bit more than lights.
Then, when we move up to the heavy bracket, suddenly we can mount every single weapon in this game pretty comfortably. You can even go balls to the wall and mount two of the heavies weapons in this game(AC20, Gauss). You can comfortably combine LLs, MLs, ACs and have respectable speed and heat efficiency. When effective customization is possible, this game is much more fun. Assaults offer even more customization, you can spend days trying out different combinations according to your personal taste. No wonder these classes are stacked.
Maybe when we get heavy machine guns, lighter ACs and other interesting stuff like MRMs and such, the mediums and lights will be more attractive. But at the moment they just cannot be as flexible as heavier 'Mechs and naturally people gravitate towards them. Speed, the main weakness of Heavies and Assaults doesn't hurt them a lot in MWO, as most matches devolve into static stand-offs. Speed Tweak also benefits them unproportionally compared to lights and mediums.
I'd say remove speed-tweak and you'll see some migration to lights and mediums. Otherwise look into fixing the utterly useless Flamer, SL and AC2 for mid-term and finally introducing more weapons that will bring flexibility to these brackets.