Why Nasujuro specifically?
Shanks obviously has charisma, elite crew and a lot of territories, and became a Yonko in just 6 years. That's a big deal.
Teach became a Yonko in two years with Gura and Yami DFs, defeated Marco Pirates in Payback War, got Aokiji on his side, indirectly causing Marineford War for capturing Ace, killing Whitebeard, infiltrated Impel Down and recruit tons of level 6 criminals, defeated Wang Zhi and take over Hachinosu, hunting devil fruits and more.
All that stuff and his bounty is almost as high as Shanks. So imagine what Shanks has to do in order to reach Yonko status.
Yonko status is not granted for free lol.