Are you a graduate? If so, is it that you are looking to go to graduate school for those subjects?
Comparing 'pure math' to 'theoretical particle physics' isn't a great idea, pure math consists of all subject areas in pure mathematics - and competition at graduate level is essentially based on the number of places that are available, and the prestige of the supervisors running the program. Theoretical particle physics is a specific subject.
And, are you talking about masters programmes? PhDs? I only have a limited knowledge on these subjects but consulting my particle theory friend has led me to say that (until we get the rest of your information! very generally): he believes because pure math and theoretical particle physics are more considered to be purely academic programmes they, by design, suffer from less funding than and so have less places, but they are interesting! so still have the applicants to make it competitive. I also know a solid state physicist that has worked on some material appropriate to quantum computing, and apparently his experience is that there seems to be a lag in the applications to the field.That is, I guess people interesting in computing are a lot in the computing sector - rather than looking at it from a physics point of view. It's also worth thinking about the fact that a lot of computing work takes place in industry.
Now, the above is second hand information and I don't have the numbers to back it up. I shall work on that now once I get the answers to the rest.