Many business users do not have administrative priviliges to install anything. And that's for good reasons.
So these users would not be able to install an alternate browser on their own.
In addition, many businesses use web based applications internally that require IE. ERP, Document Management, Laboratory Management, Process Control come to mind at my company.
So users would have to know which browser to use with which application? Or make a bunch of shortcuts on their desktop to handle the confusion. That seems very inefficient when the solution on the vendor's side is to support the #1 or #2 most popular browser.
It sounds like laziness on the web developer's side.