Susano is rare

[Like Naruto]

I think This is perhaps the rarest ability of the mangekyo sharingan because of this

the Susano means "the ability to help by all means" basically this means those only people with the mindset to help others, people who is whole heartedly loves to help people.

Madara even though his way of view was wrong he still wants to help others in his own way.

Shisui even in his death gave his sharringan because he knew there would be a time that he eyes would be helpful

Itachi he loves the village even though things have happened in the wrong path he still do love it, he help because it was the right thing to do.

Sasuke was a little different
But maybe susanoo was destiny thing indra asura but if it wasnt I think deep inside the kid sasuke doesnt want others people felt when he's clan was massacred.

Obito does have the same thing as them he though that the shinobi world is just pain theres no stopping it that's why he agreed for madara's plan even though its ridiculous

I think this is probably the reason to unlock the susano

 

[salamander uchiha]

Maybe, but the rarity may be moreso in how it varies from user to user. Sasuke's allows Enton weapons, Madara can remote control his weapons, Obito's unleashes Kamui shuriken etc.