Because Yemen is offically an ally, and it wasn't the smoking gun that the Bush White House was hoping for.

Now missiles are not particularly useful without a nasty payload, and high explosive isn't that nasty.  Biological weapons are hard to disperse.  Chemical weapons have a similiar problem, and do nothing to undermine enemy infrastructures, having use in being a terror weapon or against enemy forces.

And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg