I don't know why this is, but Nintendo seem to enjoy making it next to impossible to complete your Pokedex. First, there was Mew. Ok, in Japan, you got it as soon as you caught the other 150 Pokemon, but to everywhere else, it was rare to impossible to get a legal one.
Next, Celebi. Again, the Japanese had it easy, with the event in Ilex Forest that was taken out of translated versions.
Jirachi got a lot wider distribution around the world, coming with the bonus disc you got in Colosseum (to you US people), or from Pokemon Channel for those of you in PAL areas of the world.
Recently, the Japanese got their tickets to Navel Rock, with a 15-day event where you could download the ticket from a WonderSpot. That's 14 days more than anywhere else got for Mew or Celebi.
I think Japan just loves saying to the rest of the world 'we have it easy'.