Ubuntu 6.06 LTS ("Dapper Drake") end of life

Because that is an indication of how much work it is to support the software for so long. Supporting XP vs supporting a whole distribution is several orders of magnitude in difference.

In to that Ubuntu had to support 5 other versions of it's distribution at the same time. This with different versions of the software in each distribution. And they do it free of charge.

To that I would also add that releasing Service Packs for XP is completely different user experience from getting patches since the Service Packs often change behavior of the current software and also introduces new functionality, I would say that MS has a very long way before they can give enterprise support similar to that of say Red Hat.