1. Innovation. I couldn't bring myself to pay $18k for a used 1930's tractor engine. Builders need a cheap, viable alternative to being ripped off and I wanted to help find one. $700 for a used 30 year old carburetor? Geesh!
2. Power. I wanted more power for take-off, climb and cruise. Take-off power is a safety factor. You're in the "Oh shit" zone (Where its too late to land ahead and too early to  turn back) for much less time.
3. Cash-flow. I didn't have to shell out $18k all in one lump.
4. Vibration. Vibration is tiring for the pilot and destructive to the systems.
5. Benign failure modes. If a Lyc fails internally it often does so catastrophically. Rotaries tend to keep running, then die once they've got you home.
6. Maintenance cost. If the engine blows up, while I glide toward the nearest landing spot at least I wont be thinking "this is going to cost me $18,000 to rebuild".
7. For the challenge and satisfaction of having done it.