Sega's IPs just don't seem to have held up as well. Compare Sonic and Mario. Both the cornerstone of their respective companies. Mario has branched out successfully, Sonic not so much. Sonic is a one trick hedgehog.
A lot of their games appeal to a limited group of gamers. Total War and the Valkyria games are a perfect example. The biggest portion of their IPs are outdated and constantly released in compilations.
Their arcade presence was killer, at least until the arcades started dieing.
Sega has been such a huge part of the video game landscape that I hate to see them flounder. A turn-around doesn't look promising, but I'd love to see it.