I'm surprised no one has yet mentioned pitches and the fact that they have become flatter all over the world. Perth isn't as fast as it used to be, in fact most Aussie pitches are fairly slow and dull...the same goes for a lot of pitches here in England. South Africa has some decent fast bowler's pitches which is why we've seen the likes of Steyn and Morkel bowl 90+.
Wahab Riaz while in England was bowling at 150 on numerous occasions but he lacked the overall quality of of control and line, which coupled with dry, batsmen friendly pitches meant he got hit around the park. So that's the problem, or at least the major issue here, pitches have become flatter and bowler's have to adapt to that, they have to bowl on a line and a length to see some rewards. The art of the yorker is also limited, so it's rare to see a 90+mph in-swinging yorker up rooting stumps these days.
There are a tonne of 85+ bowlers out there, all of whom are good and some verging on being great but the age of the beast is gone, I doubt we'll see another Waqar, Akhtar, Akram, Donald, Lee for a long time yet.