Competitive play is basically the only reason that BLU is not a main job. People who do savage and ultimate prioritize the best possible group layout one can have in order to clear content. If BLU punches above other jobs with fixed skills, than those that do savage content would all switch to BLU, the devs would have to power up the bosses to make them challenging, and then everyone else has to get to the level BLU is at. The inverse can happen as well where BLU ends up getting nerfed into the floor and now suddenly it's way more effort than it's worth to play BLU, as you do not get skills the same way as other jobs.
Then there is PvP where the versatility of BLU would outshine anything the other roles could accomplish, so everyone would end up playing BLU and it would be BLU vs BLU all day.
I think the biggest hurdle BLU has, however, is that it doesn't use rotations or any kind of finger dance. It's about making a decision on what move set to bring to a fight and if you do not have that move set than you have to find a way to get it. Then, once you get that move set the BLU character has a far easier time keeping DPS up or healing than other dedicated classes because of how it's moves are not about rotations. It's like having a simplified PVP hot bar in a non-pvp setting.