Acute-care jobs are more plentiful because that is what hospitals and other institutions need. They need people who can do more than one specialty. They don't want people who can do only one specialty (even if it is cardiology).
There is no regulation requiring parity in job numbers among specialties. Employers with a wide range of work types are going to seek employees who can do a wide range of work types. People with experience in only one specialty (even if it is cardiology) are going to be competing for a smaller pool of jobs just because there are fewer single-specialty jobs (even in cardiology).