
One day, as soon as they could, Transylvania, Valacchia and Moldova joined together and became a country called Romania.
However, there was part of the Moldova area that was under the Russian control. After the formation of the USSR, a Soviet Moldovan republic was formed. And then, 70 years later. USSR fell and that Soviet Moldovan Republic became an independent country.
After having been separated for so long from the western Moldovans which had long since become Romanians, the eastern Moldovans developed their own national identity. There was a plan to join Romania, but it did not pan out.

The national identity was just part of the reason. Joining Romania would mean that their government people would now be demoted from Presidents and Ministers to governors or mayors- or be fired altogether, and Bucharest -appointed officials would be sent over to replace them. No local official would want that.
Moldova is now either the first or the second the poorest country in Europe, but they still don't want to join Romania. For the foreseeable future, it will remain an independent country.