Why did thee, thou and thy come to disappear from English? I am looking for solid explanations, rather than observations that these are still used in dialects in the north. Please explain cause for their obsolescence, if possible. It will greatly help my students if a legitimate and credible answer is provided! (Added): it would be really interesting if anyone can identify when this occurred.
My research: I have only found that the forms disappeared by 1800, but no illumination of why they fell away and were commonly replaced by 'you' as standard. I don't mean to sound apathetic, but I prefer to generate some debate or choose amongst responses from the people in the know who use this space than simply reference Wikipedia.