Why is Cavalier zimmerit so hard to find?

Because they are relatively expensive and maintaining an extensive inventory for a small online retailer would run several hundred dollars per line. Cavalier is like $30 a set and designed specifically for a particular model kit. The way many Dragon kits get updated and improved, the specific kit the set is designed for could become obsolete when the next iteration of the PzKpfw IV, V, VI and Ausf F/G/H etc. is produced. Also many of the kits the sets are designed for go OOP fairly quickly.

Then you're stuck with product no one wants and isn't worth the amount you purchased it from the manufacturer.

It would not be fiscally responsible to carry a half dozen of each set of zimmerit. They probably stock the more popular ones in quantity and then special order the ones requested infrequently.

I think Atak makes a great product form what I've seen, but they typically run an extra $10 more then Cavalier. I'd much rather opt for Cavalier for I  can find them on sale once and awhile.

Could be related to the currency situation (the AUD is currently at par or greater to the USD depending) making it more expensive for import/distribution to the US than it used to be. At one time Cavalier was as common as Atak in terms of availability but has really fallen off in the last couple of years. Pure speculation on my part but wouldn't be surprised if that was a contributing factor.