I still remember that the AF-360FGZ flash was first marketed with my ten-year old MZ-S, which was the first P-TTL flash unit of Pentax and also with the High-Speed-Sync (HSS) function, back to 2001. Four years later, Pentax made the AF-540FGZ, being more powerful with a high Guide Number and also a swivel head. After that, there is no update to those major flashguns of Pentax.
At the same time, the selling prices of those original Pentax flashguns have being increased year by year but not so for the quality. The 360 flash was made in Japan when it was first launched and several years ago, the production was shifted to Philippines.
I know that many Pentaxians have been forced to buy 3rd party flash units as a result. Meanwhile, most people have opted to buy the Metz units.
There are not many other choices for Pentax users actually. Other than Metz, another major 3rd party flash maker Nissin simply doesn't make any Pentax compatible flash. Some Pentax users just want to go cheap anyway. There are still a few of other very small brands which make P-TTL, but quality, accuracy and compatibility of those cheap flashes could be horrible. Look at this user feedback at Amazon about a cheap flash of those, for example.
Related:-
The Definitive Guide to P-TTL Flashes
Pentax P-TTL Hot Shoe Flash Comparison (External Article - Very Comprehensive, Highly Recommended)
Which Flash Units are Safe for Pentax DSLRs?