Updating a phone's operating system via a carrier Over-The-Air (OTA) update has always meant weeks and weeks, if not longer, of waiting for end users. Microsoft wants to improve this cycle with Project Milkyway and deliver updates to users sooner than before. The process was outlined during WinHEC and aims to get updates to users within 4-6 weeks of release to manufacturers (RTM). This is a lofty goal considering how the rollout of several recent Windows Phone updates has gone.
Lumia Denim was RTM'd in December of 2014 but has yet to complete its roll out on several carriers, namely AT&T. Microsoft has control over how fast they release software updates for carrier testing but they cannot control how the carrier will handle the update release process. Windows Phone having small market share also means Microsoft has little leverage with the wireless carriers to push their updates through. Companies like Apple can push updates to their phones as they please and keep their user base up to date.
Microsoft is trying to improve the process but there are still too many factors outside their control. Wireless carriers still have to "Validate OTA Updates on Retail Phones" before Microsoft can publish the update to production servers. This means the at least some of the delay which has been keeping updates from reaching customers hasn't been eliminated.
How validation occurs by the wireless carriers is unknown but that is where Microsoft could speed the process up. Windows 10 for Phones and small tablets could prove to be easier to update and test making the process faster, however nothing from has come from Microsoft stating this is the case. Luckily the Windows Insider program exists where enthusiast tech fans can get their hands on early software regardless of wireless carriers.