1) It requires a dedicated, long-term partner. If your partner isn't available, you can't play. "Pick up" bridge with strangers simply isn't feasible in the way that randomly matching teams on, e.g., an online FPS or tower defense games is.
2) The game is fundamentally broken by certain bidding strategies, which must be banned.
3) There's no way to prevent opponents from cheating by using out-of-game signals (e.g. timing of bids). This can even be done unintentionally. So fair play is always an open question.
Perhaps someone who knows more can comment about whether I'm wrong?