They might not think it's a good idea for several reasons - the fund could be too hard, or it could be too easy, or it might be too similar to another fund, or the applicant doesn't have much history in the game or on the boards. One of the things about a theme fund is that HSX is trusting you to see it through to the end, and if you haven't built up that credibility - well, good luck. There are already lots of poorly-run funds. If there are too many, then investing in funds isn't very fun, and they crowd out good fund managers too.
Looking at your profile, you have a $400M port and have grown it by 200% this year, which is shows you can build a portfolio, but you have been playing since 2000 which is not so good. Obviously you've played for a while, then gone and done other things and then come back to the game. You've done a great job growing your port this year, but what's to say you will stay with HSX and won't find another hobby? That's how HSX may look at it.
There are a couple of options - you can wait around until August or so and apply to run a studio fund, which is the traditional entry point for less well-known fund managers. Studio funds delist after a year, so the damage of a fund manager who takes an 11-month break is minimal.
Or you can hang around for a while, posting on the boards and growing your port further, and then apply again - either to run a fund based on your own idea, or a non-performing fund that isn't being actively managed.
Also, the HSX Dude wrote a column on how to start a fund a couple of years back - you should take a look at it.
http://www.hsxdude.com/7/post/2010/02/fund-week-how-to-start-a-fund.html