It could likely be a Conservative stronghold as well; it's also possible that a separate Moldovan identity had been created OTL. Or, perhaps, Bessarabia could be heavily settled by Russians in similar patterns to OTL (which are helped by the lack of demographic catastrophes), so the region has a strong Russian minority.
One thing that is helped is that the other states in the west were incidents of members of a federation voluntarily seceding from the union, and then granted support afterwards. They did not attempt to penetrate beyond their own defined borders. (the only region where borders changed were Central Asia)
However, no country invaded the IEF the establish these separate countries (well, the Technate did everything but), nor did they invade while seeking territorial gain. If Romania did invade, and in the aftermath they are busy fighting IEF forces during the armistice, it is more than likely that the IEF's initial position would be for the traitorous member of her alliance group to vacate any territory that was yet occupied before talks would even begin on the talk of recognition of the other states. The absence of a rebel group in Bessarabia shows that there simply isn't a major desire for independence, either naturally or due to the presence of a large Russian population... which, in the end, gives the same result.