I think it is a mix of lots of internally developped software which is not compatible, or at least not tested as compatible (and often this software is not developped anymore), cost constrains, and mostly a lot of bureaucracy and glacial-paced processes. I don't think it has to do with third party software.

[edit] in fact if anything third party software is forcing us to upgrade as we see some critical third party software progressively becoming incompatible with XP.