I'm hesitating to say it, but I heavily suspect that they wanted it this way -- Gnome 2 firmly in its grave.
All in all, decisions like this (whether or not to spend effort on parallel installability) are a sum of many factors, but in this case, one factor, I suspect, dominated them all.
"Diluting momentum" on a controversial change, in the high-stakes, high-competitition game of DEs.