#3 The Royal Rumble proved how popular Sasha Banks is

Just how popular is Sasha Banks amongst the WWE Universe? After her incredible performance against Ronda Rousey at the 2019 WWE Royal Rumble, she effectively helped turn a section of WWE's most hardcore fans against Ronda Rousey as was evident at how Rousey was booed on the RAW after the Rumble.
Yes, the Becky Lynch effect played a part in Rousey getting booed too, but her match against Sasha Banks was a major factor too.
#2 WWE needed the best team to win it

Of all the teams inside the Elimination Chamber, there's no doubt that Sasha and Bayley were both the most popular and had the most pedigree. They have 5 RAW Women's Championships between them as well as 2 NXT Women's Championships.
Having the most decorated team win the first WWE women's tag-team championships will only help cement the titles as important.