The v0.13.0 code elegantly introduces the features to upgrade the network right up to the point before SegWit.
The v0.15.1 code comes with the assumption SegWit and the earlier Super Majority soft-forks have already activated.

Basically, the protocol changed way too much between 0.8 and 0.15.
We can do 0.8 to 0.13 without much disruption, but anything newer than 0.13.0 will lead to issues.