“Ye” used to be plural and “thou” singular. At some point, it became common to refer to royalty in the plural, even if you were addressing a single person (e.g., the king). As time went on, this practice was generalized so that you would refer to anyone in a socially superior position relative to you in the plural.
The effect of this eventually was that referring to someone with “thou” carried the implication that you viewed them as having lower social status than yourself. Once “thou” became risky to use due to the potential to cause offense, it rapidly fell out of common usage and “ye” and “you” took over (eventually just “you”), thus causing English to lose the plural form altogether. Now we have colloquialisms like “y’all” or “y’ins” or “you guys” or “you all” to make up for this.
The effect of this eventually was that referring to someone with “thou” carried the implication that you viewed them as having lower social status than yourself. Once “thou” became risky to use due to the potential to cause offense, it rapidly fell out of common usage and “ye” and “you” took over (eventually just “you”), thus causing English to lose the plural form altogether. Now we have colloquialisms like “y’all” or “y’ins” or “you guys” or “you all” to make up for this.