What makes OLEDs optimal for flexible displays or wearable displays?
The fact that it can be manufactured by, in effect, printing it onto a flexible plastic sheet. LCDs, on the other hand, rely on a very thin layer of liquid held between rigid glass sheets, which clearly can't realistically be made flexible. Unlike LCDs, it doesn't need a backlight either as it generates the light directly.

Other display technologies such a fluorescent, filament or CRT are even less practical for reasons of size, power, cost - pretty much any reason you can think of.