Mandatory military service could cost the government millions of more dollars. If more citizens were to flood the system, there would be more benefits needed for those citizens, including water, food, shelter, and weapons. 
These things are wildly expensive.
The system for selecting mandatory service citizens would need to be developed and stream lined to notify people of their upcoming mandatory service. This new system would also cost the government large sums of money to create and implement. Mandatory service does not save the government money in the short term or the long term; it forces more people into service which will cost the government more money.