I actually do not mind that the full release of the game caps at 10th level. Here are my reasons why:

1) Most D&D 5e adventures are not meant to be run with adventurers past level 10. I recently played in a Curse of Strahd campaign and our party went and handled nearly every side quest available before taking the fight to Strahd, and we were I believe level 12 and completely wrecked his shit. He did not have a chance.

2) Larian Studios is trying to make it so that spells can affect the surrounding environment and tactical aspect of the game. Not having to account for spells of 6th level or higher will help tremendously with this.

3) Baldur's Gate 3 is including not only every class in the PHB, but at a minimum 2 subclasses each. Limiting themselves to only 10 levels means that they have more time to not only make sure they get them done right, but that don't need to spend time making content such magical equipment appropriate for higher levels.

4) By limiting themselves to a level 10 cap, it leaves them open to actual expansions that continue the story, instead of DLCs that merely add new backgrounds, classes, races, spells, and feats.

5) Finally, having a level 10 cap means that Larian Studios can spend less time on classes, and more time on actual plot and story.