Isekai has been around forever, but the specific Isekai's that has been flooding the anime scene are specifically Harem Isekai where it's some Japanese male that somehow gets transported into a medieval setting where they are overpowered as fuck, women exist to fall in love with the MC because the plot demands it, and every other guy exists to make the MC look good. I used to think Battle Shonens with "tournament arcs" were lazy as fuck, which I admit is my guilty pleasure, but a lot of the newer Isekai's basically don't even try anymore. SAO isn't exactly an Isekai, but it played a huge role in the popularity with the Isekai genre we see now.