Online poker is so popular because it is accessible for everyone. It's not like in reality, where they won't even let you into the casino table if you don't have stacks of money in your pocket. On the internet you can get into virtual tables for a few euros/dollars, because no one will kick you out. The offer is diversified, so you can choose from hundreds of online poker rooms, where they play from the lowest stakes, to amounts with many zeros.
In addition, the competition is so high that
poker sites offer substantial bonuses to those who deposit money there. Basically, in order to start playing poker on the net, you only have to meet two conditions: to be of legal age and a card with which you can make payments online.