I can't speak for Truman's health plan, but I know quite a bit about the medical profession and their relation to government medical plans.

Generally, it's about self interest.

As George Bernard Shaw once said, "A profession is a conspiracy against the laity".

While professional organisations like to portray themselves as being concerned for the public (eg. AMA concerned about Americans' health etc.) - and this is indeed the case to some extent - their primary reason for existing is to support their members. They are much like unions in this sense.

Associatipns of doctors have thus tended to oppose government health plans, as they believe this will result in doctors ultimately getting paid less.