Why do Wizard and Sorcerer have basically the same spell list, but Wiz can prepare 10 more spells than Sorc?

Why do wizard and Sorcerer have basically the same spell list, but wizard can prepare 10 more spells than Sorcerer can know at 20th level? Sorcerers are supposed to have more flexibility via sorcerery points; feedback on whether that feels balanced is pretty spotty, though

— Mike Mearls (@mikemearls) January 4, 2018