The 7 is a great phone and it’s cheaper now.
There has been a guess at the reason: people want the X. Thought is after the reviews come out and say it’s not that great, or people try to preorder and get told they have to wait until March they may give up and buy the 8.
We’ve never had a situation where there are two dates the iPhone comes out on before so we don’t know how much thats effecting demand for the 8.