Yeah, I get the in universe explanation, what I question is why this was an issue in the first place. A mech carrying around a giant pistol should be all the inspiration you need to get from point a to point b , and it's not like it's a big engineering challenge given that you already managed to modularize the thing into a pistol form factor to begin with (especially when you civilization is keeping FTL travel going with spot and balling wire... You've got to have some seriously talented engineers).
Maybe they just had really aggressive patent attorneys in the star league era? Like "on the internet" patents turned into "on a battlemech" patents and ComStar held the IP with multi century terms, while the clans were the actual successor (no pun intended) in interest... The whole battle of Tukayyid thing was actually over who owned the omnimech rights, which is why they called it a trial. Make about as much sense as the actual storyline, I guess.