Layne pointed this out to me a while ago:
The usual catch rate for legendaries is 3. Zekrom's and Reshiram's (in White and Black, respectively) catch rate was raised up to 45, way above the usual catch rate and just about as easy to catch as any other regular pokemon. Apparently, they're pretty integral to the storyline, so you
have to catch it, as Taron said. So catching it one poke ball really isn't much of an achievement, sorry to say. ;(
I haven't really done much for catching any significant pokemon except for Cobalion and Virizion, the latter of which I found to be a huge pain to capture. So I just think it's all very random; you're either lucky or you're not, for the most part, as was always the case.
Avatar: In memory of ieatwyverns <3
Thank you, Vasco <3