Before iMessage, they used the green bubbles for SMS. So unless Apple was planning a long con, they believed the green bubbles were fine for their customers for years, and switched to a new color (blue) when iMessage came about.
For some reason reading this, I didn't believe you, and imagined "black text on green bg" to be very ugly, but you're totally right. There's the classic look we remember. Hmm.
What I find ironic with all this talk of lower-class green bubbles is that the Messages app icon is still…green!
Back in my day..... all message bubbles were green!