The organizations that I have used BizTalk at (3), have chosen it for a few reasons:
- It aligned well to their corporate standards: i.e. .Net shop, run a lot of windows desktops/servers, use SQL Server
- Price, from what I have heard BizTalk is very competitive in this area.
- Generally the resources are easier to come by(relatively speaking). If not, the organizations feel that they can convert existing .Net developers into BizTalk developers
btw, I am in Canada so some these reasons may not be as valid in your area