Why? Supply and demand. .22 Mag was more popular before the 17 HMR. While popular, there are not many 17 HMR semis built, and some were prone to feeding issues. Not so much with the .22 Mag. .22 Mag has always been more of a hunting arm than a range toy; accurate enough for small game out to about 100 yards or so, but certainly not a target round, although some shot amazingly well. Mag-dumps are expensive as well.
These were very popular back in the days of the Jack-rabbit drives when I was a kid and many fox and ground hogs were shot with them also. As farming advanced, fence lines disappeared, and eventually, so did the Jacks. With the disappearance of the Jacks, the food supply for fox diminished and rabies further reduced their numbers, then coyotes moved in making fox even harder to find and further reducing the numbers of groundhog as well..
In my area the .22 mag was pretty much obsolete by the time I was as kid. The new, popular fox/Jack rifle was the .222 Remington, now it is the .223. 17 HMR is still a bit of a novelty as far as hunting goes.
Amazing to think how much things have changed in the last 60 years.