I imagine the same way they won't raise Weaponskill category cap... these Merits were suppose to be more exclusive, theres suppose to be a choice you must make based on how you play the job. In the end it usually ends up a lot of people go with a cookie cutter set up, but not everyone.

Raising it to 20 would allow a lot of jobs to cap all 4 of their Tier II merits, not all of them, but a few of them... Raising to 15 might be a bit more realistic, but I still don't see it happening, again, for the same reason they adamantly refuse to increase the Weaponskill Merit category... they want these things to be limited. Still, hearing it from their mouth is probably the only thing people will accept so maybe they'll come here and say I've got the gist of it.