Films I understand. Going to the cinema can take a lot of effort, depending on where you live, if you need a babysitter etc. TV - you sit on your ass and change channel via remote control or record via whatever device you prefer.
I'm just slightly mystified by the argument that goes along the lines of 'if they don't start promoting this show months in advance, nobody is going to know about it.'
Edit - it occurs also: the 'original British drama' type ads are also slightly different in that they're trying to raise awareness of new shows and interest people in something they haven't previously heard of. Whereas DW is known to pretty much any TV viewer - whether they love it or hate it.