I echo the question of another poster: compared to what?
They are not as popular as Glocks and other "big brands" because they don't advertise as much, and they aren't used in the U.S. military or law enforcement agencies.
The PCRs are not as popular as other CZs because the P01 is CZ's "flagship" compact 9mm pistol in the U.S. and because what little marketing CZ does is the U.S. is geared toward the fullsize CZ75s, the P07 and the P01. In order to want the PCR, you have to know somebody that has one and like it, or be knowledgable enough about firearms to know what is available and exactly what you want and why. The average gun buyer in the U.S. has never heard of a "CZ PCR" -- maybe never even heard of a CZ!
They're not as popular as the fullsize CZs because LOTS of people used the fullsize CZs in competition (IPSC and IDPA) and if you're not going to carry it (that is, if it's primarily for home defense, range and/or competition use) there is no advantage of a compact over the fullsize.
The reason I bought a PCR is because I already had a CZ75 BD, loved it and wanted something a little lighter to carry, but with the exact same manual of arms and controls as my CZ75 BD, such that when I compete with my CZ75 BD I am, in effect, practicing the use of my carry weapon - the PCR. And the P01 didn't appear to me to be a better option, because I have no use for a rail and I prefer the slightly more narrow (and slightly lighter) frame, instead.
Does that help answer your question?
Why is it their not as popular as others ?