Because to go to a clearnet website through the tor network, you have three hops. And to go to a hidden service, you have six hops.

Clearnet: You, guard middle, exit, website <-> website, exit, middle, guard, you

Onion service You, guard, middle, your rendezvous, service rendezvous, service middle, service guard, service <-> service guard, service middle, service rendezvous, your rendezvous, your middle, your guard, you.

All those hops add latency because of having to traverse anywhere between three and six different networks over the internet.