What would you suggest as the alternative? Have DISA write their own tech stack, operating systems, and cloud services? This is a freaking great deal for the government.
If I had to hazard a guess without seeing the SOW/PWS, I would bet that most of this MS contract would cover desktop support services.
DISA supports IT services for a huge portion of the DoD. This is basically a Fortune 100 company in terms of employees and scale.