etymology - Why did the old pronouns and their respective endings vanish from daily usage?

If I’m not wrong, the verb conjugation in the past used to be:

I have           we have  
thou hast        ye have  
he/she/it hath   they have

This conjugation is closer to its equivalent in the German language:

ich habe         wir haben  
du hast          ihr habt  

er/sie/es hat    sie/Sie haben

The thing is: Every verb in every tense in German ends with -st in the second person singular, and as far as I know, in Elizabethan English the verbs used to end in this way as well in the second person singular (thou shouldst, canst, makest, eatest, composest, etc.).

Why exactly did the old thou disappear, as well as related forms thy, thee, thyself, and thine, as well as the corresponding verb ending -st? Same for -eth in third person singular (it hath, maketh, etc.), why did it just disappear from daily usage?

I’m a Brazilian and my mother-tongue is Portuguese, so I’m not well aware of the background of this, and I am very curious. These forms can be found in the old King James Bible and in the poetry of William Shakespeare, as well as in many other sources, but I would like to know exactly why they’re gone, and for what reasons.

Answer

I'm sorry, but in questions of language, and especially of language change, "why" questions don't usually have answers, other than "because that's what happened".

The Early Modern English forms you give had already changed - in Old English, the pattern was very like modern German, but the distinct plural forms had mostly dropped out of use before Shakespeare's time (you still find ye goeth in the King James Bible).

The remaining distinctions are -est and -eth vs -s. The first of these disappeared when thou disappeared. The second was a dialect difference: IIRC -s was Northern, and -eth was Southern, though it may have been more complicated than that.

Edited to remove a wrong claim.