I don't know the technical reasons, but my proposal wasn't even allowed to go into review back then, and I would like to link this comment from @Chris_Lattner3 :
As Xiaodi mentioned downthread, that is not the case. Swift 4 stage 2 has specific changes that are accepted, because it has a fixed schedule and we have to prioritize the most important things for the community.
The art of evolving Swift forward is to carefully pick and choose areas to focus on, both because we want to ensure a coherent language, but also because implementor bandwidth is limited.
Beyond that, though syntactic sugar is always motivated by strong rationale and use-cases, in my…