Lately I´ve been reading Poppers "Logic of Scientific Discovery" and I am especially interested in his critics of induction as a scientific method. When he trys to show that a principle of induction can´t be formulated, he writes:
Kant tried to force his way out of this difficulty by taking the principle of induction (which he formulated as the ‘principle of universal causation’) to be ‘a priori valid’. But I do not think that his ingenious attempt to provide an a priori justification for synthetic statements was successful.
(Popper, LoSD, P. 5-6.)
Answers I am looking for would be of the kind: Popper does this, because as you can read here (insert cool link) the approach of Kant to formulate a priori synthetic statements failed.
Or: Popper does this, because he is a whatever-ist. Whatever-ists think that there are no a priori synthetic statements. Though you should take a look here (cool link again), to see why that may be wrong/stupid/outdated.
Or finally: Popper just wanted his argumentation to work, so he had to say there are no apriori synthetic statements. In fact, those statements can be done, just follow this cool link.