As already said, Goofy was the original challenge, it’s been around for 10+ years. People were doing an unofficial Dopey for years (5K + Goofy) before Disney caught on and added the 10K (I think they added it to Princess first for GSC) and making the real Dopey.
As to why someone would do Goofy and not Dopey - it’s extra days off of work, extra money (hotels, race fees, etc.) extra mornings getting up. For me, I’ve done 1 full Dopey (and last year’s Grumpy) and kind of feel it’s a one & done for me (and I think it is for most people - curious to see how the perfectly Dopey numbers drop off after this year). I did it once, I know I can do it, and that’s enough for me.