What does the critics have against this great show?

This was the best watch for a looong time. IMDB seems to agree with a rating of 8,2.

But on "the review aggregator website Rotten Tomatoes, American Primeval has a critic approval rating of 66% based on 47 reviews, with an average rating of 6.4/10. Metacritic, which uses a weighted average, assigned a score of 58 out of 100 based on 26 critics, indicating "mixed or average" reviews."

Most reviews are behind pay walls, so I cant properly read what they say, but I'm surprised by the low score.