The 4 person final definitely contributed to it. yeah we had Andrew but he was just there for the ride, once he was gone we had the 4 potential winners. And once Tom was gone we had 3 left, so it felt like we lost half the housemates since we've become so adjust to 6 person finales since 2014 (well maybe even 2013? I think CBB12 had 6 people but CBB15 had a hiccup and had 5 people in the finale... but anywho).
6 people is too much and for a finale no matter how long the show - especially when 6th and 5th leaves together (the fact we slipped back to this in CBB20

) just further supports the notion that it's a rushed, mass eviction.
I think it helps that the housemates were all varied, which Raph (of course

) mentioned. They had a late entrant, a citizen, an exile, and the People's Housemate in the final 4, and while that doesn't really seem relevant, late entrants, the divide between citizens and exiles, and Tom's status were the foundations of what this series was - I highly doubt Fight Night would've happened if Issy and Savannah didn't enter late, and the exile/citizen theme caused the mass divide which remained for pretty much the entire series, which was all thanks to Tom. Of course, the final 4 were varied in sexuality, gender, and race, which also felt really reflective of how diverse the cast was and in turn reflected how they called it "the United Kingdom of Big Brother"
Also, there were no cruisers in the finale, so it felt like a celebration of sorts? That they all contributed and made it far on their own merit, especially after they booted out half the housemates in a week?
I think the fact Metro (?) wrote an article as to why Issy should win did help contribute to the idea it was a big deal, or at least to me it did. It felt weird seeing an article as to why a BB housemate should win - which hasn't been done for years, apparently. This, and the fact Isabelle walked her win (despite, as Riley said, the fact the winner was still so unpredictable), just make the BB18 final stick out to me
And of course, the fact that all 4 would've been good winners is the biggest reason. Tom, to me and many others, would've been the worst winner out of the 4 remaining, but he'd still be an alright winner? And I really believe between the other 3, there was no 'better' winner (although I was rooting for Debs and Issy <3 )