- it isn't a priority
- it is a tricky thing to get right in the context of Go
- there haven't been any satisfactory proposals thus far
If this isn't quite right I'd love to be corrected.
In the meantime there have been several satisfactory proposals (in the lists and elsewhere), and it's not like 200 other languages implementing Generics have had many issues with them.
So, it boils down to the Go core team putting up the solution to an impossible tradeoff as the holdback (and a little exagerrated one at that, with relation to the costs involved).
And then, they said "the language won't change" etc recently.