Assuming your premise is true (there are surely plenty of players who are good at both), I would suspect it is because even though the basic concepts of poker are the same, there are important differences between live and online.

Obviously physical tells is one, although that is somewhat overrated as only occasionally does a big live hand really turn on a physical tell. (And of course, you have tells in online poker too, such as how quick a bet or raise is made.)

A big difference, though, is the ability to gain statistical information about other players. No live player can keep track of every hand. Instead, you get to know a player's tendencies based on what they are doing in the hands. And at 35 hands an hour or so, you don't have a very big sample size. So you fill in those gaps with instinct and mathematics. Maybe if you see a lot of a particular player in live games you get to the point where you know how he or she plays consistently.

Now, let's take that approach to online poker. First, since you have a much bigger sample of hands, plus (potentially) data mining and heads-up displays, the best information is going to be in the hands of people who know how to use statistical tools. You can, of course, still obtain useful information from watching what players do (and indeed you should-- a heads up display is not an excuse to ignore what how the players are actually playing their hands). But that information is going to be incomplete compared with what the computer software can provide to the player.

Further, all you see on the screen is a screen name. You can take notes about the player, though they probably won't be very detailed, but the sort of institutional memory of "I have played this guy many times and know his tendencies" is a lot easier to recall when you see a person's face rather than just an 8 character screen name.

So a live player moving online has to adjust to a world where software is more powerful than instinct and memory, where statistics can tell you more than merely observing the hands. You still need to know how to play poker, but it's a different world online.

One last observation. Online has much lower limits available than live poker. In my world of limit hold 'em, the same people who will be playing $.05/.10 on Stars will be playing $3/$6 live. $.50/$1 on Stars plays as tight-aggressive as $9/$18 or even $20/$40 at a local casino. So a live player who steps out of a casino and logs onto an online site and plays for the same stakes is probably playing better players than he or she faces at the casino.