I think this is the largest amount of legendaries Riot has given in such a short amount of time
Let's rewind the clock a short bit. Back in April, we got God Fist Lee Sin. A decent skin for the champ (though I hate the ever living fuck out of him). Then a certain leak told that we were getting some more things, some of which were another legendary for Caitlyn and two new skins for Riven and Yasuo (not named legendary though).
Jump to May and we get teasers and soon the reveal and release of Pulsefire Caitlyn. Alright, we got two legendaries in the span of two months. Nothing extremely crazy so far but still it's impressive that Riot did this.
Then we got teasers for the now Dawnbringer Riven and Duskbringer Yasuo skins and soon was revealed that those were legendaries a month later! 4 legendaries in the span of 3 months! That to me is really insane considering legendaries take time to develop and we only ever see them every 4-6 months or something like that. Maybe even a year, idk I haven't kept track of all the legendary release dates.
Now for somewhat the obvious reason, Riot did this to still make a profit because: business-business-got-to-make-profit. But on a personal level of reason I'm rather impressed. Even if 3/4 of the skins are for champs that I hate to play against, as, & don't play at all, I still have to applaud right for their effort and work on this.
(now if only they could release some skins for those in need
)