"Thou" is recently deceased, and while it hasn't been thoroughly documented, it's as likely as not due to a combination of the over-extension of the polite form

"Thou" is recently deceased, and while it hasn't been thoroughly documented, it's as likely as not due to a combination of the over-extension of the polite form of address and the use in early English typography of the "y" from Latinate typefaces for the thorn character. ("Ye" of "ye olde" is an alternate spelling of "the", and not related to the second person plural subject pronoun "ye".) The polite plural is common enough in European languages; there are French dialects (in Canada, at least) where a good rule of thumb is "don't use tu unless you're sleeping with that person or are their parent". With the second person singular restricted to intimate use, it's not hard to imagine it falling away.