Because he has the same playing style/position/role as Iniesta and plays for the same club and country.
He isn't even close to Peak Cazorla. Peak Silva is far far away from him.
It's simple. Young player emerges and performs well for some time = Next <Club Legend of that Position/Role>
Yamal was being called Next Messi by some people after 4 promising games.
Same with Pedri, "the next Iniesta".......
Also, modern Football fans are very heavily biased towards diminutive, technical looking players. Likes of Foden and Pedri will always be perceived as a level above their actual performances. While physical specimen with an ugly style of play will get rated lesser than their actual performances. For instance: Haaland, Nunez, Lukaku types.