[n.s.]

This has been done to death, but I'll chime in anyways. Here's my theories:

-Online offers much lower stakes. A newbie player who knows he not any good can jump into a $0.25-$0.50 game online, while this same player is going to be forced to play $3-$6 or higher in many casinos.

-It's easy for live players to fool themselves into thinking they are winners when they are not. Lots of them don't even try to remember how many times they bought in, how many times they went to the ATM, how much they spend on food and drink, etc... They just remember that they dragged some big pots and forget all the money they donked off. For online players, it's a lot harder to fool yourself. Depositing onto the site takes more work, there's a record created of it, and it's hard to ignore your online account balance being half of what it was the day before. I think lots of losing online players are forced to face the fact, so they either try play better, stop playing, or move down in stakes.

-I think that social pressures makes many live players more passive. Most people have a natural aversion to confrontation, so it's harder to raise someone (especially on a bluff) when looking them in eye. Just like how everyone's a tough guy on the internet, online it's easy to just click "raise" and be more of a bully. Most low-limit players play way too passive, so when they play more agressively, it makes the game tougher.

-Boredom. I think this one is a huge reason. Playing solid poker requires lots and lots of folding. Online, action junkies can just open more tables (and even at one table there is many more hands/hour). Many good players lack the required discipline to play well live when card dead. Likewise, online it's easy to just quit when you are tired or playing bad, but many live players drive for hours to get to the cardroom, so they feel the need to play for as long as they can, since they can't just log on again in the morning (I actually suffer from this sometimes).

-Alcohol. Every cardroom serves drinks, and there is a bar-like atmosphere that encourages drinking, with people buying each other shots and such (or free alcohol in vegas). Getting drunk will make pretty much anyone play badly.