Are stem cell lines cryogenically preserved or commercially available like HeLa cells are?
This is too long for a comment, so I have to write it in here:

Mostly this is because other cell lines are more practical in the lab.

Stem cells are much more tricky to maintain - especially if you want to keep their stem cell properties. They only grow very slow, tend to differentiate when they get too much stress, cellular signals and so on and need special media (these need to be defined, so that they are not containing factors which promote differentiation for example). 

For everyday experiments (like expression profiles, experiments on the effects of certain substances on the growth, gene expression or whatever of cells) "normal" cell lines are much more suited as these are much easier to keep in the lab using standard media. Additionally,experiments can be more easily repeated and reproduced, since these cells should behave always the same.