I think the appeal is simple. The appeal of streamers in general is mainly watching a live "event" so to say or live entertainment and being able to interact with the person on the screen either through chat rooms or donations or whatever. It can be very entertaining depending on what you're into. Now, there are also people who really like anime and Japanese games who really enjoy that aesthetic and the characters in those shows/games. V-Tubers are like a combination of those two things. It's an opportunity to watch and interact with what is essentially an anime character live on YouTube (or Twitch).
As to why it's been so popular recently, probably quarantine. Everyone is (or was depending where you are) stuck inside so really all online content creators saw substantial boosts to their audiences.
A V-Tuber is a person or even an actor, sometimes represented by a talent agency, who uses facial recognition software (think like Snapchat) or sometimes full body rigs that allows them to appear on screen as a 3D model of whatever they want. Usually these models are anime characters and they stream as those characters instead of as themselves.