Certain gaming houses are stylistically suited for one system's "genre" anyway. Not to mention adapatiations that must be made for consideration on certain systems.
the main reason, however, is the incentive it gives gamers to buy multiple systems. For most of us, it seems to have worked.
that's not to say I LIKE brand exclusive games, just going over why it's done.
Sega only "shared the love" because they couldn't maintain a console market and their own "brand exclusivity." They destroyed their market's faith by making console after console with minimal support. As big a SEGA fan as I was, I refused to buy a Dreamcast, because I'd seen what they did with their last like 50 systems (all released in the same 6 month period...
).
Basically, they only did it because of shitty business decisions.