Three reasons that are not exclusive and effect each other.
1- Job market is the main reason. IR and mammo where the least competitive fellowships just 10 years ago.
2- Number of fellowship spots per applicant: It indirectly effects the job market. Neuro has more spots.
3- The added value of the fellowship: If the fellowship adds a lot of value, it means that you have a skill that most general radiologists don't have. This indirectly results in better job market).
Most general radiologists feel comfortable reading 90% of Neuro studies and 95% of body, Nucs and chest studies. However, most don't feel comfortable doing high end IR procedures. MSK is somehow in between. Most radiologists don't want to do mammo.
10 years ago the competitiveness was: MRI fellowship>MSK>>Neuro>Body=IR> Nucs=chest
Right now is IR>mammo>>MSK>>Neuro=body>Nucs>chest. MRI fellowships have been phased out and are not sought after.