There are many reason for the game to be on PSN.
One of the biggest reason is for the game to be available and enjoyed to a wider audience. And it is also best in the consumer's interest(maximize the gain with the smallest spend)
It also will help the company(Sega) to gain a profit. It is a win-win situation for both parties( gamers and the company). Now that PS4 is being released, I hope Sega will consider this being released on PSN. As I for one, still want the game to be on PSN.
And for those Xbox 360 users, Sega previously had announced that if the sell goes well, Guardian Heroes 2 will be created. So please gamers, help us to get Sega to bring the game to PSN.