I'm sure all their large investors are pushing for it. Given the, uh, irrational exuberance we're seeing, they may well get more for their stake in a very liquid market, and it probably easier to buy or sell in public rather than on the secondary market.
Apart from the onerous reporting requirements once you go north of 500 shareholders, I don't see why they would bother to go public unless a very large shareholder started pushing for it. And even then it seems possible to unload multi-centimillion chunks in private exchanges.