Unfortunately this is going to be a rather short response because the subject matter necessitates it. Hopeful that will stop this answer being removed.

The disappointing answer is that there isn't a reason for this, nor really with any historical or current language change. Linguists will describe in great detail the changes which have occurred in a language, and perhaps even make predictions based on the general trajectory of a language. But they will seldom get stuck in the weeds, so to speak, trying to find a reason why because, simply put, language change is rather somewhat random. Language (or rather its speakers as a unit) does what it will with no real underlying cause.

Ask yourself, for example, why some speakers of English pronounce "th" as "f" or "d". You'll be hard pressed to find a reason why beyond "they just do".

It's also worth noting that some dialects of English did retain "ye" as a plural distinction along with "thou" and "thee", and that other dialects have innovated a new second-person plural (such as "y'all" or "yous").