You'll learn that in the world of graduate medical education (residency and fellowship), spots are often limited to funding and also appropriate training.
In regards to funding, the vast majority of graduate training positions are funded through CMS (Centers for Medicare & Medicaid Services = federal government) with a minority of positions funded through other means (i.e. state funding, direct funding from the hospital/institution). Funding covers more than just the resident/fellow salary ($40-70k/year depending on location, PGY level, etc.), it covers the associated malpractice coverage, having teaching faculty, and other intangible factors such as ordering/doing more tests, etc.
The other factor for training positions (again residency or fellowship) is appropriate training which is generally determined by the ACGME (Accreditation Council for Graduate Medical Education). For example, for an IR fellowship, there needs to be an adequate spectrum of diseases and procedures, minimal teaching requirements, acceptable facilities, etc. Based on these factors, ACGME will determine how many positions a program is allowed to have. There are, however, non-ACGME accredited fellowship positions -- both for generally ACGME accredited programs such as IR and neuroradiology and for non-ACGME accredited programs such as body imaging, musculoskeletal imaging, etc.
And just to make things more complicated, sometimes programs can be accredited by ACGME for more positions than CMS is willing to pay for (e.g. the program is accredited for up to 16 residents but only has CMS funding for 12 resident positions).
So if an IR fellowship fills and you really wanted to go there, they simply cannot just "create" another position without looking at these major factors as well as other considerations (i.e. would adding another fellow "dilute" the experience for the others? Who is going to pay for the position?).
Obviously this is a complex issue and I hope I've given a reasonable answer to your question.
