Why did projection color tvs lag?

[frenchy]

Just wondering, why is it that color rear projection sets came in to being so late in the game (the 70's)? What were the main limitations keeping even small 3-crt projection sets from coming into being, especially since they could have used 3 simple and small black and white picture tubes and shoot for much smaller screen size than what they eventually reached (65 inches)? Was it that good lens systems couldn't be built yet? The tubes still were just not bright enough and so would have been too big? Convergence would have been too complex without solid state electronics? I mean how hard would it have been to make something a lot smaller than that, like 25 or 30 inches, but still bigger than 15" and 21" complicated and expensive color pic tubes? And rectangular to boot. Even some of the the first BW sets in the 40's were projection and that was with only one pic tube.