My guess is it probably has to do with spin rates.

I was just about to look up this stat! I don't want to speculate the answer, but will watch here for others replies.

Yes, I definitely think it has to do with spin rates.   Because of their high club head speeds that they generate, it causes a lot of spin so they want to keep that down so they use the harder ball which is the prov1x. 

It spins less than the pro v1 off the driver but still spins great around the greens and approach shots. 

ProvX is for faster swing speeds. A regular prov1 is too soft for a pros swing. The spin rates and the softness would be too much for a professional to use. Prov1Xs also go a bit farther and have less spin

I would guess spin too. I'm sure most Tour players have a high spins rate, and the 1x spins a bit less.

If (from the main Titleist homepage) you click "golf balls", then "golf ball fitting", then "education"... you'll see that swing speed is NOT the determining factor for choosing the right golf ball.

Pick your ball from the green back to the tee, not the other way around. On an average par 72 course, you're taking at least 58 shots with putter, wedges, and irons... but only (at most) 14 with the fast moving driver.

The ball has to work for EVERY swing speed and shot.

the Pro V1x adds more distance and does not get as much spin as a normal Pro V1. With the amount of spin most pros get they don't need the spin so they want the extra distance. Hope this helps!

X more spin than the 1...I love the Pro V1 for my swing speeds,high 80's low 90's.

higher ball flight and less spin.