OK, here goes: "Because I said so, that's why."
I bet that doesn't work, though I have said that before.
I've always considered time in grade a troubling thing for just the type of situation you've described. I've been "guilty" (?) of violating that paradigm myself, once in kyu grades in TKD, HKD and Aikido... and again in dan ranks in Aikido.
For me it really is a troubling thing to say, "Hey, I know you're competent enough to be X rank, but you've only been at the material for 6 months, so we're not going to promote you until you complete the 2 years time & grade. Yes, I know you can compete witht hose others of X rank, even some who are past that, but hey, rules are rules, so we're going to hold you back."
It is a thing which bothered me, still does. Then again, lots of times people "think" they're ready when they're clearly not, which is a different issue entirely. Whose communication about expectations needs fixing?