Well because it requires SS-Rank and the rules describe it as not being able to "Attack, defend, heal or buff." and as nothing more then "something neat."
So it's incredibly difficult to obtain and hardly useful.
We have three choices here address one problem or the other or a little bit of both.
We could keep it's limited power as is and reduce the rank requirement to say B-Rank or C-Rank and then it'd get use.
We could up up it's power to being a second secondary magic but just knock the rank down to S-Rank(because SS-Rank is a weird rank anyway.)
Then finally my ideal would be to make it a second secondary magic that you get at A-Rank. In this case to balance it players would only get spell slots up to the rank right below them to keep it weaker then secondary. I.E. an A-Rank player who just got it only gets spells to B-Rank.
As for mentoring it(which should be the case since it would become a second secondary magic), the option for S-Ranks to train themselves should also be given. Since there is a distinct lack of H-Ranks walking around. It's one thing for secondary since they unlock the option at a low rank but since we'd be unlocking tertiaries at A or S Rank finding a mentor would be really really hard.
And really really hard things get under used and under appreciated.