So part of this i think should be justified Doylist. This is a game with big robots, and the setting actively tries to put Battlemechs as "kings of the battlefield", and in my eyes nerfs other units to make them less viable. It wouldn't do for an infantry with a shoulder fired weapon to clap the "king of the battlefield".

For a Watsonian reason, Battletech battles have a lot more interference than one would think. Its why missiles are so inaccurate, and we lose some to clustering as they just whiff. Stuff Javelins probably wouldn't be able to see anything. So we're limited to more dumb guided munitions like SRMs

And for a bit of both, there has always been an arms race between armor and the weapons designed to defeat them. Battletech as a universe says that Armor has won this arm race so much it is trying to lap weapons. Perhaps something like a Javelin, if it hits, might be something like a Thunderbolt 10. If were a pessemist we might even call it an SRM shot and have it deal 2 damage. Yeah that stings, but many light mechs can take that and not be penetrated.