They'd just lose money releasing a new hardware... I mean, sure Sega has many 'orphans' but the fan base is just not enough to make sure they'd profit.
Also, I heard that, 'till last year, both Sony and Microsoft were losing money for each X360/PS3 sold, and, since this year they're not losing money on them anymore...
Sega lost a lot of money for each Dreamcats sold back in 2000, and we all know what happened... their finance/capital is not as big as Sony's and Microsoft's...