Maybe because not everyone wants their spreadsheet with sensitive data on Google's servers. Also, I seriously doubt this thing is up to feature parity with Excel.
Because Excel is the backbone of how most organizations do "computation". For example, if you look at any financial services company, you will find not only Excel everywhere, but tons of VBA code in those spreadsheets which also turn around and talk to various compute clusters that they use (both Windows HPC server and LOTS of Linux). Excel isn't going away anytime soon.
I've worked with one organization that uses Excel sheets to model product mathematics for gaming (gambling) equipment. They won't even use the Macintosh version of Excel since they can't be sure the math will come out exactly the same as the PC version, and the PC version is what has gotten them approved by the gaming boards before.
Call it superstitious, but they don't have the time or expense to sit there and prove that another spreadsheet works 100.00% identally to PC Excel. So now they run Excel in VMWare on OSX. There's irony there, but what can you do?
Most people I know who sit in Excel all day use Windows Excel on a Mac. OS X is a better platform (it's nice to have the terminal right there) but Mac Excel is trash, and Microsoft knows it.
having worked at a multinational for some years where excel was the bread and butter: privacy, power features (e.g. tables, goal seek, etc), and custom-built (corporate) plugins are the three biggest reasons I can think off.
