Its an American internet web anime, something that has been tried a lot and has crashed and burned just as many times. Seeing it done semi-competently is something people had never seen before. Mix it with Rooster teeth being at the height of their popularity + some very well done trailers and you have a hit.But again they're really nothing special that hasn't already been done a million times before by shitty paint by numbers anime trash.
Besides Monty's fight scenes tend to look like someone bashing action figures together.
I think people are really overstating how popular Monty was. His name and popularity was really niche before the RWBY trailers where he started popping off. You can even see this by the shoe string budget and lack of animators the first season had that RT didn't have a lot of confidence in the project until they started seeing the numbers the trailers + first season generated and poured money into it for season 2 onward (which did help for volumes 2 & 3).
RWBY was better in volumes 1-3, but never good. It had passion though, which good or bad is something most people admire and subconsciously seek out in media. Similar to the Room, which is a trash-fire, but a passionate trash-fire that many people can't help admiring in a way.