Well the simple and possibly biased first glance answer would be:
CMU just doesn't cut it for the top 10 programs in mathematics. Their PHD in math is not a PHD in financial engineering, its a PHD in Mathematics with a concentration/dissertation topic in finance. If you want a PHD in mathematics with your research to be in finance I'd recommend applying to schools with top PHD programs in mathematics AS WELL AS a top business/finance/MFE school/program.
Mind you I haven't looked at their PHD in math placement, but judging from your disappointment this is probably the reason.
Schools like MIT, Princeton or Berkeley would be examples of places to do a PHD in math AND and research relevant to FE/CF. They would most likely have excellent faculty to prepare you for doing research as a math PHD while at the same time you'd have have access to top business school/mfe program fauclty to also advise you. You might want to consider the possibility of doing a terminal MFE concurrent with ( if the programs are flexible enough ) or after your PHD program, which isn't entirely unheard of.
CMU's CS is its most reputable dept, not mathematics.
The only caviat would be to make sure that someone ( or more) would be able to advise you well for your PHD in the area of your interest prior to dedicating 5-6 years of your life to research.
And on the topic of dedicating 5-6 years of your life to a PHD and $50-100k for an MFE...you might want to be absolutely sure that research in FE/CF is something you want to do.