Ok, so I'm going to preface this by saying, that I am just giving my opinion and two cents. I am in no way an expert on anything regarding why different specialties are harder or easier to get into. Perhaps, we can call out Wahoos to answer this one.
However, this is what I think:
I think that fields like Derm and Radiology are competitive because soooo many people want to go into those spots due to their 9-5 nature for good compensation.
On top of that, those two particular fields have less available spots for residency, making it it competitive.
Finally, in the case of those two fields, there are not as many of them as there are of family physicians so that might have something to do with it.
Last but not least, they are procedure based on many occassions allowing for better compensation, and hence making it more popular then IM or family med.
IM is competitive once you get out of residency and are looking for subspecialty fellowships.
This is my thinking, but I admit ahead of time that I am no informed expert.
Wahoos and Harvarti,
If you see this post, please come out and give your two cents.