I think it appeals to the same itch that languages like Brainfuck scratch.

There's something exceedingly interesting about how you can model complexity with something extremely simple. Brainfuck is fun because it forces you to think extremely low level, because ultimately it is basically just a raw implementation of a Turing machine. I wouldn't want to write a big program in it, but it is fun to think about how you might express a complicated algorithm with it.

Similarly with CGOL, it is really interesting to see how far you can stretch really simple rules into something really complex.

I've written CGOL dozens of times, it's a common project that I do to "break in" a language I've learned, since it's not completely trivial but it's simple enough to not be frustrating, and I completely understand why math/computability-theory folks find it something to dedicate brain power to.