Because "The only way to stop a suicide bomber is a shot in the head" - quote from somewhere on the net.
This was actually said by the chief commisioner Ian Blair.

The best way to stop a suicide bomber is to find out the root cause that make people feel so powerless that their only means of protest is by blowing themselves up. We need to find out what causes people to be pushed to such an extreme. Find that out and address the cause and you don't need to shoot anyone in the head and reduce the likelihood of someone turning to suicide bombing. Give someone a reason to live and they will choose life. Remove any reason for living, then who knows how they will choose to die.

But dealing with the causes it probably too much like hard work. It's always easier to deal with effect, rather than cause.