I often wondered the same thing that you're wondering. I've always built hot rods and I also drag race, (1:1 stuff). I've always been an on-road type of guy in my full size rigs, never been into off road or 4x4 stuff.
In RC I like off road stuff because I don't have a track to race on-road. Once I got into RC prety heavily, I didn't understand the appeal of rock crawling. I had around 30 RC's, all built to haul ass. I finally got the chance to run a crawler on some small rocks for a lil while, and I was into it. I actually built my first crawler thinking I would probably sell it if I wasn't into going slow.
That was around March, and I've been hooked to crawling ever since. I had a blast with my first crawler, and soon was building a comp rig and getting into competition. I now have about 4 or 5 crawlers and two complete scalers in the works, with parts being gathered for scaler 3 and 4.
Rock crawling is an experience like no other. It's the only type of RC that you'll find where every rig is unique. In racing, if there's 15 buggies on race day at a local track you're doing pretty good. Most guys will all have similar mods done to their rigs. In rock crawling, there are no two rigs that are alike. It's all about personal setup and driving style, and if you give 100 guys the same exact parts, they'll manage to build the crawler at least 99 different ways.
Rock Crawling gives you the same rush that running a race rig or basher does at 60 mph, but it's also very relaxing at the same time, and you can do it in a 4 foot by 4 foot area while drinking a beer. I can come home after a hard day at work and chill out in the lawn chair and practice with one of my crawlers for 30 minutes and be completely relaxed. At the same time though, its a constant rush every time you're trying to balance on a line and come with a few mm of a gate without touching it.
Also, you can build a nice crawler for a very inexpensive price, and if you like to build then you can spend months adjusting and fine tuning your setup to get it dialed in without spending much money at all. If you aren't really into building that's ok too, because you can throw your rig together pretty easily and have a blast with it just like it is. When you're hanging off a ledge with your sideways and completely vertical, and you have that intimate relationship with your rig to know exactly where each and every balance point is and where the limits are, its just as invigorating as launching off the line in a 8 second drag car to me.
If you have any questions feel free to shoot me a PM. I've spent countless hours with lots of the members here answering their questions and helping educate them on what the options were before they bought their crawler so they didn't make poor decisions, and I will help anyone that I can get started in crawling. I'm definitely not the most experienced veteran, but I can usually either answer your questions or point you in the right direction. I take great pride in the fact that if I don't know the answer I'll send you to someone who does, rather than making something up and giving you bad information. Just let me know if I can help you out at all.