Because when you blow up anything orbiting, for example, 200 Kilometres above the Earth it turns into a million little pieces of shrapnel speeding around at 28,000 kilometres per hour, thus ruining things for all the other astronauts, more or less permanently. So:
Since debris cannot be effectively removed from orbit, controlling the production of debris is essential for preserving the long-term use of space Banning anti-satellite weapons would also improve international security by preventing the erroneous interpretations of satellite failures as deliberate attacks by enemy states.
Given its knowledge on space security, Canada is well-placed to take on an international leadership role by enthusiastically supporting initiatives for the drafting and ratification of a an international treaty on the prevention of an arms race in outer space.
Canada can also do more. There has never been any comprehensive, coherent and overarching Canadian national policy. Instead of a piece-meal approach, Canada should have a national space policy that is adopted after thorough and public debate in order to govern and guide the conduct of Canadian space activities in an organized and sustained manner.
Certainly, whispered hints on the part of the Harper government that they might support the latest incarnation of Star Wars don't help.