Right as the game arrived for Early Access, players started noticing the MLB The Show 24 XP Cap seemed significantly lower than in previous years. However, with XP bunched together as program rewards in several areas, most players couldn't quite pinpoint the exact value. In the last MLB The Show 24 update, developers have addressed and explained the Diamond Dynasty XP Cap.
MLB The Show 24 XP Cap Explained
We now have confirmation that the MLB The Show 24 XP Cap for Gameplay XP is 15,000 XP per day. This does not include XP earned in other ways, so completing other programs in Diamond Dynasty will still provide chunks of XP that increase your progress towards the Season 1 rewards.
Developers at Sony San Diego pushed back on fan frustration around the XP Cap by explaining that it's necessary to properly pace Diamond Dynasty content "to prevent the 99 overall rewards at the end of the path from interfering with the gradual rise in player overalls too soon." Essentially, their goal for the XP Cap and programs appears aimed at preventing a few lone players from pulling too far ahead of the pack by grinding Gameplay XP in bulk and getting 99 OVR players early.
This also ties into their push to move much of the Season programs into other areas. There aren't any Season 1 missions this year, as in the past you might earn a chunk of XP in the Season 1 program after finishing a Conquest map or adding a recently acquired boss to the Season 1 Collection.
Most of that is still happening, but they've moved it. Instead of XP rewards found in the Season 1 program, they're baked into the Conquest rewards and earned as you progress in the Spring Breakout program or the upcoming Egg Hunt program.