Bottas has a good track record and strong tires with Toto Wolff at Williams. They knew Bottas and trusted him to be capable at what they wanted him to do.
Rosberg wasn’t as challenging to Hamilton as many people seem to think he was. Hamilton always had the upper hand. In 2017 Bottas performed on par with Rosberg, 18 was very poor, and 19 has also been on par.
Both have their strengths and neither can challenge Hamilton on pure skill.