Why won't the new CV joint installed stay in it keeps popping out

Too many variables here but you need to start by making sure the axle is all the way into the trans. There is a little clip at the end that needs to snap in. A hands on inspection is needed. https://repairpal.com/repair-shops Alex

Engine is 'drinking' oil .. Transmission needs rebuilding .. Axle shaft popping out of place .. Perhaps it's time to consider purchasing a newer vehicle..