im no expert myself but the reasoning behind the downgrading I believe is because newer firmware such as 2.5 and 2.0 have more protection against code it dosent know (homebrew) so I think downgrading just makes it easier to run homebrew (1.5)

In think probably if there was a way to run them "cracked" as you say it would have already been done.. but its not that easy im sure.. and it dosent help that not too many developers aer working on psp homebrew etc..