It may sound low but reminder it was on HBO a payed for network. Beyoncé's Life if But a Dream aired for 1.8M viewers and that was a record for a documentary in over a decade.
Out of interest, how are TV ratings? Declining like album sales or remaining constant?
No idea in general, I believe ratings have gone down a little recently all over as people are not watching shows live anymore and recording them. However this is a pretty standard HBO special rating
"The biggest star in the world" should be pulling Game Of Thrones numbers...
A bit of reading comp in the OP would show you that this is a pretty normal HBO special number. Remember Beyoncé's 1.8M ratings was a HBO special highpoint in over 10 years