3 reasons (of many):
Let's start with recency bias and the continued failure of 200 Hockey Men. Marc-Andre Fleury had a SPCT close to .950 going into the Final. The assumption was that he'd continue this, despite the fact that his career playoff SPCT was under .910 (it's now .911). He was below .890 in every game in the Final was barely above .850 in the Final. I'm honestly shocked how many people thought that he had somehow become that much better and had somehow re-invented himself (this does not happen). If you flipped a coin and it landed on tails 10 times in a row, over a large sample size it's going to be roughly 50% heads and 50% tails. They teach this basic math concept.
While Vegas was a strong possession team, you need to look deeper (warning-analytics explainer coming). What Washington did VERY well was 2 things: dominate the crap out of teams at 5v5, and generate high danger shot attempts. Vegas dominated getting low-danger shot attempts (think from the points or from the blue line), but those attempts didn't result in goals (and they normally don't).
Ovechkin. Simply put, he is, in every possible sense of the word, better than anyone else and getting shot attempts in the league, and it's not even close (he's in Gretzky/Lemieux territory when you start adjusting for era meaning if you put him up against the flaming piles of crap that were NHL goalkeepers in the 1980's he starts producing 140-180 point seasons). He's been doing this since the 2005-06 season. He's a shot-generating machine, and he scores goals. It's almost like hockey is a sporting contest whose object is to score more goals than the other team. Meanwhile your 200 hockey men talk about compete level and that he "wants it" which is narrative-driven garbage.
I'd also add Vegas playing facepuncher Ryan Reaves. Grandpa on CBC might like him because Grandpa (Don Cherry) is senile and probably wears an onion belt, but he's a liability on the ice (if he's hitting people it means they don't have the puck which means he's playing defence and he's not particularly good at it). People pointed to the goal he scored in game 1, and ignored the fact that he got caved in during games 2-4 before they finally benched him. Putting Reaves on the ice opposite Ovechkin is like trying to keep up with Ovechkin's drinking on that 5+ day bender he's been on.
I'll give credit to Micah Blake McCurdy, whose analytics model (@ineffectivemath) had Washington winning from the start (she picked Washington back in October). She also nailed Pittsburgh winning in 2016 and 2017.
Winnipeg/Nashville "should" have been your West Final. I had Tampa/Washington and Nashville/San Jose as your Final Four with Tampa and Nashville in the Final.