According to user reports on Reddit, the EX Raid system in Pokémon GO is still experiencing problems that Niantic hasn't solved yet. The latest one being the lack of players needed to conquer the system, as the user in that link notes, he was the only one to answer the raid and take on Mewtwo, which he eventually lost because it's a hard battle. This is due in part to the raid invite bug that's been plaguing the system since it started.
There's really been little word from Niantic as to when they plan to address these issues. For now, those playing Pokémon GO are basically rolling the side to see if they get an awesome raid or a dud. For those of you who have succeeded at it, awesome job! You are one of the few.