Basically the way you grow stem cells is extremely complex. Stem cell cannot grow "alone" and must have a monolayer of cells, usually fibroblasts, underneath them.
Secondly they grow extremely slow. It takes 7 days to see a colony of stem cell.
Next, you have to keep stem cells undifferentiated. Afterall, it is this property that makes them special. Unfortunately they love to differentiate every opportunity they get.
Finally, you have to feed them everyday so they maintain healthy. For bacteria, you leave them on a plate for whatever days and they still grow. But for stem cell, they just die or differentiate (which becomes unless).