[@UnknownScarlet4] Oh! One I can answer!

They are neko, and they are also maids. So they have the appeal of both. 

Longer version:

Maids have been a fetish staple long before anime got a hold of it. The uniforms are cute, can be sexy, and often worn by a servant. Most people who appreciate maids either enjoy the outfits or the fact that the maid is a servant. It lends itself to a control fantasy where the observer can place themselves in a controlling roll. "As you wish, master."

In anchient Egypt cat-like behavior was thought to be sexy, but Japan's nekos made the fetish debut again in recent times. While originally strictly sexy characters, neko have started moving in a more "cute" direction. while not easy to control or predict, they are usually stupid, which can also lend itself to a sort of control fantasy by "outsmarting" the neko.

As you can see, these fetishes just barely overlap. This is probably why neko maids aren't as widely appreciated as their vanilla counterparts. For those who do enjoy this combo, it's as simple as liking both cat parts and maid uniforms. Or perhaps they simply like having greater control over their servant.