From the preiviev of Hades operation:

As stated above, only soldier units can enter the metro and the sewers. Additionally, these units must enter the battlefield by the end of the second round of the game or risk being left behind. It is akin to suicide to get stranded in the underground, and any unit in an underground scenario that has not been deployed by the end of the second round is considered to be eliminated.

OK, so how in god's name can i not move my units into the BF in the first round? There is such an option even?

 I'm guessing here but if the map is only underground then deployment space may be very limited. Some maps may only have room to deploy a handful of units or less so if you can't fit your army on the board you have to leave some men behind.

Most of the included scenarios allow you to deploy above ground with entrances throughout the board so it shouldn't be do big a deal. If you design a straight underground scenario, I would suggest setting up multiple deployment tunnels or setting some of the large rooms as deployment zones