Genuine answer from my side:
Because the only buildings on my island which aren't lv6 are those which can't be upgraded beyond level 5 (aside from the Nobles, which are still longing to be replaced with Towers) and because the only resources which I still need are Exwood, Titanium and Saltpeter - since they don't grow on my island.
And because I'm already running 10 buffed Iron Mines 24/7 and it's a major annoyance spending about 2 hours/day ...
... and maybe because I've purchased Premium for real money and don't see why I should give BB free money for nothing I can use just because they provide inferior service!