While there are traditional (brick and mortar), casinos in many major cities and in every region, they do not excite potential poker players nearly as much online poker does. This is due to the following reasons.
Traditional Casinos Can Be:
Intimidating: For novice poker players or for those individuals that are not particularly impressive with their skill sets, playing other people, especially those who may have superior ability, might be intimidating. Internet poker is not as intimidating. You do not have to expose your real identity and you do not know the other people that you are playing. This makes it much easier to risk getting your handed handed to you. You are also probably more willing to stick to it and put in the time and effort to improve because you can do so from the privacy of your own home.
Embarrassing: Some individuals have qualms about going into casinos. They may not want anyone to see them visiting or leasing a casino, much less actually playing a game. This is not a problem when you are playing internet poker. You can play whenever and to your hearts content because no one will ever know.
Expensive: Traditional casinos have much higher overheads then online casinos. As a result, the minimum bet requirements at the table are much harder. Online, you can start playing a hand for as little as $ .02. Also many online poker sites will match your initial deposits, double your later deposits and give you bonus money. This allows you to play without as much financial risk and also for much cheaper. Some online poker sites allow you to even play in tournaments without an entry fee. So as you can see, playing poker at traditional casinos can be much more expensive then playing on the internet.
Since the early to mid 1990's when poker became available online, the popularity of the sport has absolutely exploded. Individuals that would have never considered playing the sport, are suddenly giving it a try and are even becoming fans. Internet poker, in many ways, makes up for some of the games weaknesses that people often bring up when discussing playing poker in a traditional casino. Namely, that the game can get expensive, it's too risky for novice players and it can be intimidating. Online poker cuts down on the intimidation because you and the person (s) that you are playing against, do not know each other, or at least can not see each other, you can start off playing for free or at least a significant discount. There are also really great playing specials that you can only find online. This is again, because online casinos have much smaller overhead costs and can pass the savings onto you, the player.