Two decades ago no one really cared about potential draftees working out for scouts and personnel evaluators for a team. Then the NFL decided to broadcast their annual combine that occurred a few weeks after the Super Bowl because they had a network that needed programing. People watched, and now every sport has a combine that also serves as part evaluation process and part show for players hoping to be drafted.
MLB was perhaps the last one to really go deep with this, but a lot of that was the function of when the MLB draft was. The draft used to be held while the majority of the NCAA Tournament had yet to be played, and so you were looking at a situation where a good chunk of the players just couldn’t attend because they were, you know, busy.