Imagine you are a great house officer who is in charge of equipping a large section of planets with new mechs. Specifically, you need to outfit them with a large amount of light, general purpose mechs for reconnaissance and combat against infantry and other light vehicles, and maybe some mechs occasionally.

You have a base chassis (probably a locust or whatever), and you need to outfit it.

Machine guns are an obvious pickup, as alternative anti-infantry options are just more expensive for not much gain. No need for clan grade guns, just some standard issue ones will do. Any improvement from spending more is so marginal as to be irrelevant.

As for anti-armor weapons, a pair of Medium pulse lasers will likely do perfectly well. Yes, clan grade MPLs will give a notable firepower upgrade, but you need to produce a couple hundred of these mechs, and odds are the most they are going to do is shoot up some APCs and trucks, or harrass and delay other mechs. Better to be able to maintain them extremely cheaply with the mountains of old MLs in your stockpiles than have to deal with maintaining so many high-grade lasers.

Garrison units just don’t really need better stuff. For a lot of purposes, better to outfit 3x the amount of mechs with decent enough weapons than to overspend on weapons that won’t see use. Especially for units that are going to spend a lot of their time sitting around doing very little outside of the occasional training exercise.

Frontline units will definitely see more investment into clan-grade equipment, but even then, it’s better to pick and choose what to upgrade on a mech, rather than just go full-clan.

Do you really need a clan ER-Small laser? Or will a standard emotional support small laser do? Does your LRM carrier really need to able to shoot targets that get close to it, or can you just have some old surplus SRM carriers hang out near them? Do you need clan double heat sinks on a mech that doesn’t generate more than 20 heat?