That's expected given that ShB and EW were the culmination of a decade-long story, while DT is something new. I like to use the baseball analogy that not every batter can hit a grand slam, because doing so first requires three other batters to get on base. Similarly, not every installment of a series can be a 10/10. Those are the end-product of a much longer build-up. DT is the start of that new build-up, and a future expansion that ends up being another ShB or EW will be able to do so precisely because of installments like DT. Me personally - I'd rate DT a 7/8. Yes, that really means 7/10, but from a practical perspective, I feel an "8" is the highest it could even hope to attain given where it lies in the series.
So, we know a certain element of the gaming population is out in force review bombing. Good to know.Slipping from one of the worst rated xpacs to one of the worst FFs all time. Yikes.