I should say, 'the main reason'... there are other indications as well... like 'progress'.
And you touch on it - 'preparedness'... but that's only the start. Some say 'adjustments'... but I think that term isn't quite right for McVay's approach because there is an implication of a somewhat static approach when using the word 'adjustments' ... that is... a 'game long' strategy 'adjusted' periodically... I don't think that's McVay.
While McVay certainly enters each game with a 'plan' the 'plan' is not static... it's dynamic. Situational awareness is not based on 'halves', or 'quarters', but rather on each drive... each play. AND - his situational awareness applies not only to the Rams but their opponents as well. He is as aware of his opponent's strategy as he is his own. Obviously Wade is in play here as well.
But even there it's not what makes McVay's approach to games truly remarkable... other coaches are similar in awareness and dynamics... but what separates McVay is that like only a very select few other NFL coaches McVay not only is intricately aware of each 'moment' but he is
ABLE TO BE... IOW..... his team is implementing, executing, his, and for lack of a better phrase.... 'seat of the pants game strategy'. Bad phrase, I know, because it implies something less than 'well thought out'... which is NOT the case with McVay.
Like you I don't know... talent still rules and I'm not sure the Ram's have the talent to conquer the NFC... but in what I just wrote I believe, really and truly, McVay and the Rams may be the best in the NFC, and just possibly the NFL.