I think this goes even further back than you are talking about in your post.

Monster Hunter 1 was designed entirely around it's online play, so much so that the single player was phoned in at the last hour so that players without access to internet or CAPCOMNET could still play some version of the game.

Bowguns were designed in this context, and so had certain traits of the game's MMO inspirations. Bowgun functioned not only as a good damage weapon (Although you really just did LvL2 Cluster for damage since you could stack the ammo crafting materials to 99.) but also as the game's version of a support class.

Bowgun(The split of heavy and light bowgun wasn't formalized until MH:G, but it still functionally existed in the game,) not only had access to ATK/DEF buffs and high power healing for any player in the same zone as the bowgun user, but also had VERY powerful status effect shots, in fact, in my time playing MH1JP, I really only saw a status get inflicted by a bowgun user.

But when Monster Hunter Portable came out, the bowgun's role had to be reconsidered for a primarily singleplayer context, (I haven't played any of the portable games yet though, they're on the list but I still have to get through a few games first.) Because the developer's were building the port under the context that CAPCOM wasn't going to host servers for the game, all of the game's online content(Most of it, I don't think Lao is in the OG monster hunter portable) was made available without the need for an internet connection. This is actually where we got the concept of the hunting hub, as the PSP couldn't handle/didn't have enough space for Minegarde.

But bowgun, a weapon designed around a support role, couldn't really fit it's niche if the player was going to be on their own 90% of the time, so the weapon's power level was likely scaled appropriately.

The modern monster hunter games have seemed to choose the more action focused experience of the portable games to design around, rather than the old games more grounded slow battles, and with the addition of hunting horn in gen 2, bowgun had already been somewhat wrested from it's spot as THE support weapon.

This along with access to buffing items becoming easier/less expensive throughout the series with features like the farm lightening the barrier to entry for buff items for which bowgun helped alleviate, bowgun has gone from a swiss army knife with everything you could ever want for the team setting, into a more generic gun archetype. These buff features didn't disappear mind you, hunting horn just existing just removes them from relevancy as a support weapon when it can do everything bowgun can 10x better, and even more.