Good question.
I recently read an old interview asking that question of Sharp, which was the first company with an edgeless screen on a phone a couple of years ago.
They said that the main problem was that a much thinner frame along the edges meant that thicker glass would be needed to keep a large phone rigid, and that would distort the display.
Apparently that problem had to be solved, although they did not say how they had done it. I am guessing that a thinner OLED display allows a stronger hidden frame behind it. And/or they had to wait for stronger thin glass.