Couple of reasons.
1. The look. To me, the TJ is what a Jeep Wrangler is supposed to look like. It's not too boxy, it's got some round parts, has the iconic grill and headlights...
2. Extremely capable while still being mostly mechanical. Much moreso than the following JK. It's relatively easy to work on (like the starter, easy to replace) and parts are pretty cheap.