Flat black is super easy to paint and dries almost instantly, so it's easy to do a decent paint job outside without stuff getting in it and when it gets damaged, it's very easy to touch up and match (the downside is turning gray, but I find any kind of cheap paint protector can renew it for a couple of weeks).
That said, I feel there's a large dearth of creativity in a lot of the custom scene. Some people do some amazing and clever stuff, but for each of them, there's about 50 that are build-by-numbers. I never got why someone would go through so much effort and time to make something their own only to finish it so that it looks like 50% of the customs based on the same car. Flat black, flames, two center stripes over the top (especially on Cobras!—it's like seeing a completely different car on rare occasion I see one without the stripes and I'm a little over an hour from FFR, so I see a lot of them). I get the reasons for the popularity, be it ease (flat black, flames to cover problems) or historical reference (flames, outdated national racing colors), but when there's three of the same at their local Cars & Coffee, why not go with an alternate? Like, I have about five paint schemes in mind for the boat I want to build and even though there are no similar boats to encounter (high beam:length ratio, sliding canopy, tail fins), I'm still staying away from popular color schemes (like Gulf racing colors, even though I like them) or too obvious ones (WW2 aircraft, which I'm guilty of overdoing myself due to this obsession that I was born with). Do a lot of people just have this singular, unalterable vision, I wonder? Sometimes I wish I had that as indecision about options is one of the biggest things that slow me down. I guess the upside is that I'm less discouraged when things don't go according to plan and have to change.