Chris M aside (which is 100% correct), the reasons I also see are

1) Easy rules, complex play
2) Massively mentally challenging
3) The most hopless can win big sometimes
4) Everyone liked the old poker (western) movies but never really knew how to play. Being televised changed all that

5) The INTERNET! - people don't have to go to casinos and card dens, which have a seedy image.