Let's begin with a fact: about all the RPG or MUD use turn-based system. There are few exceptions (like the Amber™ RPG for example).
Turn-based means synchronous system and non turn-based asynchronous system.
Asynchronous systems are a mess to manage. They are hard to code and even more hard to understand when they run. Nevertheless, they seem closest to our perception of reality. It's also more flexible (at least because synchronous systems are a sub-class of asynchronous ones) and it's said quite adapted to object designed entities (like distributed agents behavior).
Synchronous systems are easy to implement a coherent and persistent time system. They are compliant to the largest number of RPG systems. They are better for equity between players (connection speed or lag are less important), and between players and AI (reacting faster than a one million times faster AI is really hard). They also have a high reliability.
It can seem that a turn-based system will be more complex to set up than an event-driven one, but that's only at the beginning. Dealing with asynchronicity will require to put more safeguards, and it will become more difficult to foresee what could possibly happen. We are here talking of thousands of Entities here, not just five or ten, and the complexity'll reach an unmanageable threshold.
For all there reasons, we choose, at least for now, to use a turn-based system.