I heard an argument on NFL Network that the trend in the proliferation of receiving tight ends is largely due to the NFL rules that protect defenseless receivers now. That makes some sense -- seam and deep middle routes are a little less dangerous to run now that the receiver is less worried about the safety missiles that used to blow them up. (Although I will say VD does take some big hits on his receptions still).
But the athletes like Gonzales and Gates really opened coaches eyes to the potential, and sense then you've had the VD, Graham, Gonk's, and Hernandez's join the athletic marvels at the position. Shannon Sharpe was really the predecessor of these guys, but in the 1990s style of offense he was not a typical TE fit for teams.
I guess it's kind of a chicken or egg debate.