Yes, they just stuck with it because of just works mental, but actually nothing wrong as long as it is work and also the way that platform works is fair. Metatrader is one of the defacto platform because this platform works well and you can easily trade in foreign exchange without much problem.
Why for someone to have "just work" mental? It is saving time, and also if your proprietary platform proven to be more secure, faster and have more option to deal with foreign exchange trading i think that is not a big deal. Some proprietary platform preferred because of their unique feature which it has and has better performance too.