The show doesn't have much in the way of a budget (or RT is just exceptionally cheap) for voice acting. The voice cast is mainly one of two groups of people.
Rooster Teeth Employees - this encompasses the "main" cast - nearly 70% of it, but these people are not VAs by trade. They are high level people (like VPs) down to secretaries - although a few people have left the company since. This also includes the husbands/wives/friends of various employees as well.
"Famous" people - Famous is in quotes because the people are web famous instead of real famous. This included Jessica Negiri - but also includes "some guy from Achievement Hunter who knows Miles" or "one of the guys from Mega64 but the 4th one people forget about" or "a guy with a fedora from Team FourStar". These are people (even Ngiri) who probably do it for cheap/for free and cannot command a VA's salary. It's likely that RT is trying to draw some fans out of these people's audiences.
Starting from Season 4 forward - they started hiring "real" VAs, but in small roles just to boost their profile. For example, they hired Tara Platt - but she voices Blake's mom in like 3 episodes and will never be heard from again. They got Chris Sabat to play a villian who doesn't talk much (or scream at all). Yuri Lowenthal voiced a very small time villian who they won't likely go back to. They had Vic in a semi-close to main role, but for many seasons was just a background/side character.
They're basically trying to do this as cheap as possible while still trying to grab some names (Sabat, Lowenthal, Platt) that people care about, but not putting them in "major" roles. Any new characters that are going to be major parts of the story you can count on them being RT employees, youtubers, or really cheap/new VAs.