So let's say you've got a Successor State. At the top, you've got the Chancellor/First Prince/Grand Poobah/whatever. Not only does this guy have a lot of money personally, and probably owns a large private army, but he's also got a hundred different titles, and a million different alliances with various nobles.
Grand Duke Doofus who runs the Backwater Conglomeration, a group of a dozen planets in the most remote part of the kingdom, is happy to send five Battlemech regiments to help his third cousin twice removed (by marriage), the Successor Lord. And so the Backwater Hussars are considered a vital part of the Successor State's armies. But they're gonna make use of that weird Crusader variant, because it's produced by Grand Duke Doofus' factories. It doesn't matter if there's a better variant out there, this one is the "best" because it's produced by the Grand Duke. And by God, you're gonna make a purchase every year of two dozen Battlemechs and send them to units under your command, because that's part of the 700 year old treaty that originally joined the Backwater Conglomeration to your kingdom.
This kind of thing isn't unique. It's the standard way of doing business. Everybody's got their own factories, and produce their own style of mechs. Yes, to a degree combat ability matters, but there's a whole lot of redundancy in design because you've got to buy from the right person.