New specs like Grid Layout do take a lot of time in browser implementations but that’s not because the browser developer delay it, it is but mostly cause the spec keeps on changing(evolving).
The implementation and syntax keeps on changing as W3C irons out the spec, it is only when the spec is in near perfect stage that the browsers start implementing it.
As for the CSS Grid spec; it has came a long way since its proposal by Microsoft in 2010 and the current version of the spec is very different from what was proposed initially.
The actual implementation of the CSS grid spec started towards the end of 2014 and the latest working draft is from 19th May 2016. Considering the changes in the spec, I believe it is a pretty good progress by far.
Most browsers have got the grid enabled under a flag so a more stable implementation is just around the corner now! :)
Extra: You should check out this talk by Manuel Rego who is one of the guys implementing the grid spec in Blink and WebKit, he talks about the spec and some technicalities involved in the implementation.