I've given this topic some thought lately, so it's kind of neat to see I'm not alone in noticing this trend.
One of the factors here is that composite sticks really haven't added all that much to slapshots. A few years ago, the show Sportscience had Shawn Thornton (yeah yeah, not the best test subject) take slap shots with wooden and composite sticks. There was no difference at all. Maybe if you put a 2014 stick in the hands of Shea Weber there would be a bit more improvement, but there's only so much that can be done to change the physics of a slap shot. On the other hand, wrist/snap shots have changed a lot since the introduction of composite sticks. The amount of flex that guys like Kessel and Ovechkin get on their wristers would have been impossible to achieve before composites were introduced.
With stick technology closing the gap in slap/wrist shot velocity, shot quality becomes a more important factor. How often does a player truly get the puck in his wheelhouse for a slapper, so that he can exert maximum force and accuracy? My guess is, probably not THAT often. Usually there's some sort of pressure involved in releasing it quickly, which reduces both velocity and accuracy. Again -- this closes the gap between slap shots and wrist shots.
The single biggest factor in whether or not a direct shot will beat an NHL goalie is the timing of the release. Is the goalie set and ready, with a clear view of the shot? Or is he moving and trying to track it through a screen? There's a reason that even in the shootout, the preferred way to score is to get the goalie moving and surprise him with a quick release. If an NHL goalie has time to get set, even on a shot from right in front of the crease, he'll still force the shot wide or gobble it up the majority of the time. So there's a premium on a quick release, which actually justifies slap shots in one situation -- the one-timer.
Other than one-timers, taking the time to stop a pass and wind up a slap shot simply defeats the purpose. It gives the goalie more than enough time to square up for an easy save, regardless of the shot speed. If there's no screen, a slapper is often barely better than a turnover -- especially if it caroms around the boards in the other direction. If there's a screen, then it's preferable to take a softer shot and look for a deflection.
Basically it all comes down to a trade-off between velocity, timing and precision. Before composite sticks, slapshots provided a degree of velocity that could justify losing timing and precision. But today, the velocity gap has narrowed to the point that slapshots provide little advantage, yet they still have their drawbacks. With the exception of one-timer situations, a player who chooses a slapshot is almost always making a tactical error (even if it sometimes works out for him, as in the horse-and-buggy analogy).
