I think it is because the game only has 90 players per organization. There is not much wiggle room to bring guys up. 15 players would be depleted from your minor league teams and most of them would come from AAA. In the game, they don't have the resources to allow that to happen. Besides, AAA teams are in playoff battles too. It might be too hard to do the AI in determining which players get moved up and such. Most teams won't call up their 40 man players if they are in a playoff battle.
I think the Show needs to boost from 90 organization players to at least 100. For people who enjoy the in-depth side of it, it would be great.