I was surprised to see so many gaps in the crowd, I thought it would be close to sell out. Rugby Inside Line has recent attendance figures:
2012: 81,779
2013: 81,703
2014: 81,193
2015: 80,589
2016: 77,109
2017: 79,657
2018: 75,128
2019: 75,329
2020: N/A
2021: N/A
2022: 72,784
Whilst attendances have been falling I don't thing many extra tickets will have been sold once the finalists were confirmed. Sarries are nobody's 2nd team with many rugby supporters actively disliking them, not just for the salary cap issues but because they are an unpleasant and unsporting team team to watch.
I wonder how many extra tickets would have been sold had Quins and/or Saints made the final instead of Sarries (depending on how top 4 finished) even against Tigers, and I think Quins v Saints would have been a near sell out and a better spectacle for ITV to show to the wider public.
There's also an element of Twickenham being an poor experience place to go because of costs (someone said £6.50 for a pint in the comments!) and poor transport links, but for a final I still think many rugby fans would make an effort to see a good game.