Mega Man Legends won't be coming to the PSN as a PSOne classic due to licensing issues. According to Capcom USA VP Christian Svensson, Legends "failed an I.P. sweep and a contract check."
When asked by a fan at Capcom's ongoing "Ask Capcom" series of videos, Svensson explained that a number of games in the company's library use intellectual properties and voice acting performances to which the company no longer holds the licensing rights to use.
As such, Capcom avoids re-releasing classic games due to fear of potential litigation. In other words, you should pin the blame on the lawyers if you ever get mad over the fact that you'll never have a chance to play Mega Man Legends on the PS3.