A service that involves the repeated sending/receiving of data, (in this case email) incurs a cost over time that someone has to pay for.
There are three solutions to this issue:
A. Run a free service until you run out of money. (not sustainable)
B. Run a free service subsidized by ads. (user-hostile and possibly unsustainable)
C. Charge the user a subscription fee. (fair and sustainable)
Unfortunately, the perception that email and other services should be free has permeated society. The internet made the transfer of information cheaper by many orders of magnitude, but it is still not free.