Last year, he was only in recharge events only and after that he stopped showing up at all for a few months. Then sometime when great tree became to show up a lot, Hokage Minato reappeared again as a redeemable and started to appear in recharge events and lucky board/missions then became to show less frequently then he started to show up in RNG events where you could have gotten 80 frags. If you start to see this trend and any other ninjas. they will disappear from events so they can give room for newer ninjas. This doesn't mean they won't be gone forever but you can't expect it to show up quite often as you expected.

This is why the next time you see him as a redeemable, you better have enough to finish or get the full 80 frags since he won't be appearing as often as someone a little new like kakuzu earth grudge