Can only talk from my own experience - and it all boils down to the bottom line, litterally.
I have heard arguments ranging from 'not company policy' to 'too expensive, everyone will want one'.
At my present employer the standard machines is a 32-bit Win7 with a max of 3GB of RAM. Having to develop locally means having Eclipse, a db-server and a web server running. So with all the other 'crap' and email aso. running, my machine is having a hard time and I do a lot of waiting.
To solve my missing second monitor problem I have settled on installing a virtual desktop, I still have to switch around. But structuring what you have where helps.
Not that the above has much to do with monitors ;-)
To err is human - to blame it on a computer, even more so. - Robert Orben