It isn't about ammo availability, 99.9% of the time, as noted above, it is about target damage.
I run a couple monthly rimfire matches here in Southern Colorado, one a belly match and the other a PRS/NRL match.
I have had a few people flip me shit because I don't allow 17s. I explain I don't want my targets getting destroyed. After I tell them I cant afford to spend a grand or two converting my mild steel over to AR-400/500, they get quiet.
If I had more than a few people asking for it, I would phase out mild steel and start a 17 Division. But the demand isn't there.