The main reason for holding out in the Baltic states was getting the troops from Russia and the Ukraine out - while circumventing hostile Poland. This operation was finished in March 1919 because rail traffic in the Ukraine was no longer safe. (There remained an isolated outpost at Nikolayevsk, which later was evacuated over sea by the French.)
After the end of the evacuation, there was no reason to stay - and only some Freikorps remained (partly asked for by the British), but these Freikorps soon were at cross with the national governments - and eventually had to leave as well (after the British had changed their mind).