It was a combination of factors for me (a "hobby" rancher with both the wife and I working full time town jobs to support our ranch habit):
1. First and foremost, they are extremely low maintenance. As a complete newbie to the cattle business, I need all of the help I can get from the animals. Many folks refer to them as "weekend cattle" because they require so little care. Having to do a 24-hour watch to help pull calves, with the potential to lose a percentage of calves and/or calves, wasn't something I was interested in. The breeder we bought ours from has raised longhorns for 24 years and has yet to pull the first calf.
2. They are efficient and effective foragers. We have 45 acres or so of pasture that was neglected for 10 years before we bought it. Longhorns will clear that out better than most anything but goats, plus I can run more per acre than most breeds if I want to scale up.
3. There is an increasing market for grass fed lean beef, at least in this area. Longhorns will get docked heavily at auction if you go that route, but that would be a last resort for me. I already have several people interested in buying freezer beef when it's available. Plus there is a lot of money to be made at the registered longhorn sales, if your cattle have enough horn. You can also sell or lease them as roper stock and you can sell the horns and hide in addition to the meat.
4. They are very docile, despite their reputation. After two days, mine would come when called and follow me around as directed. The reputable breeders cull heavily for disposition - the last thing you want is a bad attitude attached to a 6 foot set of horns.
5. They're gorgeous and have an infinite number of horn and hide color combinations. People just love to look at them.
6. I don't know if this holds true everywhere, but every single person associated with longhorns that I've talked to has been extremely nice and helpful. Everyone is someone that I would like to get to know better.