I think there are three main reasons.
First, the elites usually do not want to resign their powers. For example, East Germany leadership opposed the reunification to the end. TheHistorically the elites historicallyare much molemore likely to support secessionism than unification.
Second, as Wladimir Palant pointed out, unification with Romania requires abandoning any hope to restore Transnistria as a part of the country. Transnistria in theory only agrees to reunify under condition of no future NATO membership of Moldova. This rules out the possibility of incorporation into Romania.
Most people in Moldova know Russian and do not know English. Conversely in Romania nobody knows Russian and many know English. And, more importantly, knowledge of English is required for official documents because Romania is part of the EU.