That was a factor too, forgot to list it. Although the "hippie pacifist" part is certainly a little exaggerated.
What is sure is that the Ubuntu belief system, and the UEF government system, while working like a charm in the confined borders of Sol, is inadapted to rule, economically, the much vaster territories of the GTVA. And, on the military front, the UEF, while not "pacifist" by any meaningful extent (you don't build three Solarises and superior fleet bombers for policing actions), isn't ready and/or willing, from the GTVA point of view, to make the sacrifices required to ensures the survival of humanity under a constant Shivan threat.
The ultimate goal of the GTVA is defending themselves against the next Shivan incursion. Had the UEF been committed to this objective in a comparable manner to the GTVA, a pacific solution might have been considered. Since it was not deemed the case, the GTVA is back to square one : conquering Sol to get their hands on the enormous industrial and logistical value of the system, in order to consolidate themselves in their original goal.
EDIT:
When the Sol portal was first opened, probes launched and data from the UEF public communications received, the Tevs had analysts working days and nights to figure out what was the best course of action to follow, from political, military and economic points of view. Keep in mind that their solution was originally to seize control of Sol in a single, masterly-organized manoeuvre with the 14th battlegroup, not to engage in a long, costly, system-scale war. I am pretty sure that, had the GTVA known the situation was going to escalate the way it did, they would have had to consider other options. However, after the AoA events, it was too late to come back on that decision.
