Super Mario Kart had 20 courses(only 8 unique stages, with the other courses being variants), 8 drivers, and was limited to 2 players. The driving, items, and AI were very limited, and all of the courses were flat due to it being a Mode 7 game.
Mario Kart 8 has 32 courses(16 brand new and 16 reworked retro course), 30 characters, customizable vehicles, bikes, gliders, etc. which all effect how your kart drives. It has numerous items which didn't exist in SMK as well as the new anti-gravity feature. The way karts handle in newer Mario Karts is completely different from SMK, which also didn't have features like drifting, drafting, tricks, etc. Not to mention online play.
It's not a series with huge innovations every game, but it's definitely changed significantly from its starting point.