Despite carrying such capacities 3 to 4 times Suburban makes such losses. This gives a clear view of how low its fares are. And even I have traveled in it, so know their pain and troubles. But these low fares are responsible why there are hardly any facilities. Just compare a ride in Metro and MEMU to know the difference. However crowded (Yellow or BLue line DMRC) but they are still comfortable as they have seats only on side and max people stand, while for MEMU coaches have more seats and less space to stand. Whiles fares are higher Metro does not run empty.
As for Sleeper classs they can simply fix a rate say 50 paise per km (D>200km), include...
more... SMB for more WL clearance and issue non seat just travel tickets to help day time passengers who have to board SL coaches due to overcrowding in General bogies.