Some of the skills may break the balance of the game.
For example, if you raid as Blue Mage, you might be asked to bring certain skills and learn them if you're missing them.
Yoshida would kick you if you're missing certain skills!
So for these reasons, that's why Blue Mage is more solo-focused.
Blue Magic Level 5 Death would be... a little bit broken.
Do they consider giving bosses invulnerability to things like Level 5 death? Do they consider restricting their access to content? That'd kill the fun of the job.
It was difficult to implement Blue Mage... until now.