Many reasons:
1) Thousands of miles away, not our problem, bigger problems closer to home.
2) War weariness, after Iraq and Afghanistan the British public and government really aren't in the mood for any more boots on the ground intervention.
3) Boredom with the media banging on about Syrian refugees all the time. Even the Express has to vary its Diana/Maddy McCann/'worst winter in 100 years' stories every now and then.
4) Boredom with morality warriors telling us that we should all feel guilty for not giving the refugees our clothes, boots and motorcycles.
5) The emergence of ISIS related terrorism in Europe over the last couple of years really hasn't done the Syrian plight any favours.
6) It's gone on for 5 years or so now and turned into a stalemate, people are just bored with the whole thing now. It's not NEWs anymore.
7) It's not like a movie where there are good guys and bad guys, all the sides seem to be bad guys. What kind of plot is that?
8) Brexit :P