My guess is the added distance and lower spin vs the Pro V1

I would say that Most tour players on the PGA  play the PROV1X  because this is the best ball for there game.On the Top ranked LPGA the PROV1 has a slight edge.Again I would say it all boils down to which ball fits there game.

Thanks for your input, but the opposite is true.  The ProV1x is advertised to be longer with less spin and a harder feel, whereas the ProV1 is softer with more spin.  That is why I find it odd that most tour players prefer the longer ball (supposedly) over the ball with more spin & control around the greens.  

The Pro V1X has less spin off of the driver then the PROV1 If you look you will find there is only a 500rpm difference between all Titleist golf ball's hit with the driver and that all Titleist golf balls offer low spin rates off of the driver.The true game changer is in the short game the PROV1X  has more spin Then the PROV1.

In truth, the Pro V1 gives you a bit more control on the finesse shots around the green.

Check out this article for some more insight on what the pros are looking for in their golf balls when they work with our experts on tour:

Titleist on Tour: Golf Ball Fitting with Fordie Pitts

http://www.titleist.com/teamtitleist/b/tourblog/archive/2015/05/19/titleist-on-tour-golf-ball-fitting-with-fordie-pitts.aspx

Two sleeves of new, plus a few "gently" used loose in the bag. All Titleist of course. I typically keep a few previous year's version of Pro V1x's in the bag, just for those times when at the end of the round I need a ball and I don't want to bring out a new one for the last swings.

Paul

One of the distinguishing factors between the two models is the dimple pattern, which affects the lift and drag of the ball in flight. 

That triangular dipyramid dimple pattern is money!

Gary, at the high swing speeds most PGA Tour players high spin rates off the driver is often an issue.  The ProV1x has a lower spin rate.  A few want a lower spin rate on short approach shots, so they don't back it up too much on shots into the green.  The ProV1x accommodates this as well.  Conversely, this is why LPGA players and most amateurs find the ProV1 a better fit.  We want and benefit from the additional spin the ProV1 provides on approach shots and chips and pitches around the green.