To answer your question: because they didn't watch it.
Grade "A" Fully Loaded
"Sexy as Hell"

John C Reily and Will Ferrell have this special dynamic whenever they perform together. I'm sure they had a blast on set during filming. I enjoyed seeing those two act a fool with these characters.

𝕍𝕀𝔻𝔼𝕆-𝕊𝕋𝕆ℝ𝔼-𝕍𝔸𝔾ℝ𝔸ℕ𝕋

I FIND MYSELF BETWEEN YOU AND THE CRITICS/PUBLIC..I ENJOYED THE MOVIE,HOWEVER IT IS DEEPLY FLAWED AND NOWHERE NEAR WHAT IT SHOULD HAVE BEEN.HOLMES & WATSON STACKS UP WITH SOME OF FERRELS OTHER NOT QUITE RIGHTS…SEMI-PRO,KICKING AND SCREAMING,ETC.
“I do this real moron thing, and it's called thinking. And apparently I'm not a very good American because I like to form my own opinions.”

My password is password.

Wow that's too bad you feel asleep. I was laughing too much to fall asleep.

I love Will Ferrel & John C. Reilly. When they're on it they can make you laugh til you cry. This one though, a snoozer. I literally fell asleep 30 minutes in.

Oh well. I enjoyed it a lot.

I love taking jokes. Especially when they're funny. This movie was pretty devoid of those particular types though. It was almost Dane Cookian.

Learn to take a joke. People are so uptight when it comes to Sherlock Holmes and his sidekick Watson. That's what makes the movie so freaking hilarious.

To answer your question, because it wasn't funny.

Yes I read the harsh criticism for the film, but I wanted to see for myself. I'm glad I did. I really liked the movie. It was hilarious.

No, prize goes to hellboy
Well, this is a tomb. I'll make them feel at home

It also rec'd the 2019 Razzie Award for "Worst Picture".

As a huge Holmes fan I really wanted to like this film but found it to be a huge disappointment. To be fair, I'm not rating or reviewing the film because I bailed after 20 mins. But here are some quotes from the critics:
This Will Ferrell/John C. Reilly parody of Sherlock Holmes is so painfully unfunny we’re not sure it can legally be called a comedy….ROLLING STONE
Holmes & Watson is one of those movies that goes beyond unfunny and into a comedy-cubist zone, where jokes are no longer recognizable and laughter is philosophically impossible….NEWSDAY
Puerile, imbecilic and imbued with the kind of casual 1970s sitcom homophobia that reads all male friendships as somehow suspect, this slack-jawed grossout comedy represents the nadir of Conan Doyle adaptations….THE GUARDIAN (UK)