With starting from scratch it was more important to have a stable base from which to start than to get all the shiny new features. At the time this work was started 7.1 was still very much in active development, and to some extent that is still the case. I believe it is unfair to say that 6.1 is 2 years old as it is still getting bugfixes as little as 2 weeks ago. The changelog was also examined and the small number of features of 7.x we were interested in were easily backported. As @ChuckPa pointed out part of the work done was to ensure that future updates are measured in weeks instead of months which should lead to more frequent updates.
