At one point during my time in the military, four different variations of our field/working uniform was authorized for wear while on duty. As the uniform design, features or material were upgraded, the old uniforms were still authorized for wear. There are several reasons for this. One reason is that uniforms are made in bulk and a goodly supply of older style uniforms, in some cases, were still in the supply chain. No further uniforms of that style would be manufactured and as the supply depleted to a pre-decided level, that uniform would be 'decommissioned'. 

Any individual who still had that 'model' of uniform in 'serviceable' condition were authorized to wear it. An exception to this was that some units or circumstances precluded variations in uniform appearance such as drill formations, parades, certain positions or posts, etc. where everyone was required to 'look alike' or wear 'current issue' (that is what 'uniform' means after all).

Starfleet being a military organization has similar protocols. That's why you see different 'era' uniforms on different people but the bridge crew because or their position would always be 'well dressed' with the most current issue uniform.