In this case, AMEX may have taken a look at their total exposure across all cards and the algorithm set the limit based on that. I don't know what your monthly Platinum spending is, nor is it any of my business, but that could be a factor if it is substantial.

Best advice I have is to make as many payments as you need to so as to run through around $15,000 a month through the card for a few months, then request an increase. Rather a pain to lose the float by doing things that way, but I suspect this will help getting the $5K limit raised.

I would also take a look at my credit report to see whether there are any things in there that aren't right. Such a low limit compared to your other lines would raise a red flag were it me in your shoes.