Is there any reason I am only allowed to Commit to 3 proposals?
You can commit to more proposals—it's just that first, you have to fulfill your current commitments.
In particular, two of the three sites you've committed to are currently in public beta. If you ask/answer some questions at Quantitative Finance and/or Personal Finance and Money, those commitments will then show as fulfilled, and you'll be able to commit to new proposals.
Because of the effort you'll need to put in visiting the site, voting, answering and asking questions to fulfil that commitment.
Now multiply that up by 3 - should all three go into private beta at around the same time - and you won't have much time for anything else.