In collecting aspects.
Nintendo has cardboard boxes. Cardboard boxes are hard to find in nice shape and in most cases thrown away.
Sega cases are hard cases more chance of survival. Also I find the nintendo boxes to look better with their colours but the sega ones also do have some charm to them.
Allot of nostalgia for people is what Nintendo gives thanks to some notable charaters wich other consoles do not have especially mario and zelda and later pokemon. Sega does not have such notable charaters. (sonic?) Sure but the sonic games are mediocre most people focus on the mascots to base if the games are any good for a system
Mario and zelda games are excellent while sonic games are pretty mediocre ignoring all the other great games the sega console had to offer wich are forgotten and not popular thansk to this even though excellent games comparable to snes.
These days, Sega makes really bad games mediocre sonic games and other ****. The only notbale newer games wich are retro now are jet set radio future and panzer dragoon orta upcoming shenmue 3. Nintendo however still makes excellent games and even kids recognise this and start collecting for the older system thanks to the awesome charaters of nintendo mainly mario, pokemon and zelda.
The good sega games wich are forgotten are pretty expensive these days because gameplay wise excellent however forgotten thanks to the sonic games wich defined the system for most people talking about sega: mega turrican, castlevania bloodlines, musha etc etc.
Early Sega games that came in the plastic cases were very nice. It's always easier to find complete Genesis games because it wasn't as easy for kids to utterly destroy those hard cases as it was to smash a cardboard SNES box. Later Genesis games switched to those crappy cardboard boxes that I think were
worse than SNES boxes. They had those little tabs that held the cartridge in it's spot and those got bent or ripped pretty easily.
I'm not agreeing with you on Sonic games being mediocre. Sega put a lot of faith in those titles and Yuji Naka and crew made some fun, fast paced platformers there. The series was highly regarded and in fact, was a prime leader in Sega competing with the SNES. Of course taste in games is subjective, but they were highly rated and a major factor in the success of the Genesis/MegaDrive.
Modern-day Sega started off strong when they went 3rd party. Panzer Dragoon Orta, ToeJam & Earl III, Crazy Taxi 3, Skies of Arcadia, Phantasy Star Online...these were all good, strong games. Unfortunately, last generation saw them peter out. I'm not a huge fan of the Yakuza games, but they are still fairly highly regarded.
I can't really speak for Mega Turican or Musha, but the reason Castlevania Bloodlines is so expensive is because
every Castlevania game pre-PS2 era is expensive as hell.