Temp you can control, pressure you can't. Knowing the temp is too hot allows to back off until you've seen it drop a bit, or drop right down, whatever you feel happy with.
Pressure knowledge is only needed to know if you have it, or you don't. A simple red light works just as well, and would lead to far less worrying from owners who don't really understand it. How many people have we had here over the years worrying about their oil pressure, when all they need to know is that anywhere between on the scale is perfectly fine?
Granted, that same group of people may well panic if the oil temp hits 110c or whatever, so you can argue the same applies. Still, I know what I'd rather have.