Why do scholars think that the cities of huari and tiahuanaco were affiliated either by trade or religion?

The major reason behind such thinking is the "Artistic Style" which is common in both of these cities.

Huari is a historical civilization in Andean Region which was from approximately 600 AD to 1000 AD while Tiahuanaco in the same region refers to another culture in between 200 AD to 1000 AD.