I have heard that Houdans are not easy to raise -- less robust health, pickier about environmental conditions, less fertile (both in terms of # eggs laid and in terms of egg hatchability).
I do not know for sure if this is true -- if it is, then that right there might have something to do with their scarcity -- but certainly the reputation *alone* would discourage me from wanting to mess with 'em myself.
The other thing to bear in mind about 'rare breeds' is that there are an AWFUL lot of chicken breeds (especially if you consider 'worldwide' not just in north america), many of which are fairly similar to each other anyhow. Given that there are many more people who don't have chickens than those who do, you would expect a lot of breeds to be rare just from the luck of the draw, sorta, you know?
JMO,
Pat