Why is Microsoft Retiring NT 4.0

I am currly an MCP (Microsoft Certified Professional)and stying for my MCSE. As I play with this OS all day I wonder WHY they are retiring WINDOWS NT 4.0. Surely they are NOT going to stop implementing and dial up ANYTIME soon. Big companies are STILL with NT Including mine, and migrating to Windows 2000 is not happening any times oon.