Couple of things on yorkers:
A) I hate commentators and ex-players going on about yorkers, yorkers, yorkers, implying that modern players are imbeciles for not bowling them every ball. Let's get that out there in advance - I'm biased against yorkers just because of that know-it-all attitude haha
B) Modern batsmen play the yorker a lot better than batsmen of the 70s, 80s and even 90s did. I'm not talking so much about the arsey scoop shots, I'm talking about things like simply backing away to leg so you can hit the yorker with power. MS Dhoni's helicopter shot (yes and the scoop shots...) is a great example of batsmen now having a plan for yorkers and practising different shots to counter them.
C) Yorkers are hard to bowl, especially in isolation - easier to get your range with a few in a row. That's why the yorkers I remember usually come in clumps, when a bowler really gets it right and/or he realises the batsmen is clueless. I think of an over Malinga bowled to David Warner a few months back in Australia - all wide yorkers beautifully executed, and Warner had no skills/plan. But going back to point B, most batsmen these days have got a plan or the skills to get away yorkers, especially if you know they're coming.
With enough practice yorkers can be improved for sure, and this is one of the key things. A guy like Malinga gets that practice - he doesn't have to play Test cricket, and plays heaps of short stuff. If he can't bowl a yorker with consistency, then no one can. Dirk Nannes arguably should have been able to do this as well, but I think once he realised that yorkers were too hard for him, he focused on other things. It's not going to be the same for every bowler, so expecting them all to be able to bowl yorkers on demand is ambitious.
D) I wonder if analysis isn't playing a part here eg. analysts will have a look at all yorkers: successful and attempted, then determine how successful they were and compare them to other types of deliveries. Good yorkers are great balls, but bad yorkers are knee high full tosses, and that disparity makes them a risky delivery, one that analysis may have determined isn't worth the risk. Perhaps players are getting advised, if you can't your yorker right at least x% of the time, don't bother.