Why don't we have theater-specific (vice service specific) uniforms?
The simple answer is that the Services are responsible for Title X issues, which is everything involved with manning, training and equipping the force.
The Services are worried about world-wide deployability of their force, so they don't consider many Theater specific issues.
The Combatant Commanders are worried about theater specific issues, but with the exception of SOCOM, the CCDRs do not have the ability to procure equipment.
Canada went to a single uniform for all it's forces years ago and went away from it about 2 years later for a few reasons.
First, you lose Service identity. If you don't think that is an issue, then I challenge you to wear black shoes with your khakis to your next AOM.
An additional issue is if you consider when you walk into a Joint HQ and need a question answered, who are you going to look for? Most likely it would be someone who is wearing "your tribal garb". If you have an aviation question, you'll walk over to a guy in a flight suit. If you have a question about Marine Corps stuff, you'll look for someone wearing a Marine Corps uni.
So, why do we have different uniforms? It's because the Services want their own uniforms. They want to have something that is unique and clearly identifies members of that Service.
As for the problems with uniforms, the Army tried to do a one-size fits all. It didn't work in Afghanistan.
The Army going away from BDUs and DCUs meant that the Services couldn't just just buy into the Army's uniform contract which is what had happened for years. Since the Services had to do their own thing, they decided to do something unique. You may agree or disagree with that decision, but that is why we all have different unis now.