I am simply guessing, but I reckon Ruger still makes the Mini because it is "compliant" with many States' firearm laws, and since many States are poised to restrict Semi-Auto firearms, Ruger thinks they will be able to sell quite a few more.
The newest Minis are, from all accounts, quite respectable as far as accuracy goes, and also very reliable. Historically, the price for the Mini has always been relatively high, compared to the AR, with some exceptions. Certainly in the days when the civvy AR was uncommon, the price for the available Mini was preferable to the almost unobtainable AR.
IMHO, the drawback to the Mini is that it requires the user to use great care and proper tools when cleaning the bore. The use of a coated cleaning rod AND a muzzle guide is mandatory, lest the muzzle of the Mini be damaged, thus adversely affecting accuracy.
Nowadays, things are different, but the AR is slowly being banned in many States, while the Mini seems to run under the radar somehow. Perhaps the lack of an "evil" pistol grip and no bayonet lug favor the Mini over the Black, EVIL AR. Not always, though. Times are changing, and soon every self-loading rifle will be challenged. Mark my words.