Fallingover - because it can be a hassle to light the fire every time you want to use the room, it might be that you only want to spend an hour in there, by the time you've gone to all that hassle, got the fire lit, got the room hot, it seems hardly worth it. My neighbours have an open fire, she admitted after the first excitment of having it, now 18months after moving in they light theirs only about once a month. She can't be arsed with cleaning it out and setting it, and then keeping the DCs away from it, much easier to click on the heating.
Plus it has the effect of making that room very hot, but depending on the layout of your house, not warming the rest of it. So you may well end up running the heating as well so bedrooms, kitchens etc aren't icy if you've got all members of the family in different rooms. If you're going to have to pay for the heating anyway, why bother with the faff of the fire?
(BTW - I'd love a real fire, I'm well aware however, that our gas fire has only been on twice this winter as when it's cold we'll put the heating on, and I don't know anyone who rents who's got one, few landlords are going to want that hassle)