22 WMR is a great round but the problem is that it is not as cheap or as readily available. The 22LR is probably the most widely available small caliber available anywhere. Then comes price.
A box of 50 rounds of 22 WMR is about $7-$10; a box of 550 rounds of 22LR can be had for around $10. That is a big difference. Even someone on a very tight budget can get a 10/22 for around $200 and 5000 rounds of ammo for another $100.