Because then if you have two blokes it will turn into one of those awful bromances, which end up completely ruining the show. Lost count at the number of shows that I have loved, and ended up hating because people cannot stop shipping the same sex characters together. 
Anyway hasn't the companion always been a woman in this?
They just need to find another Tom Baker and Elizabeth Sladen. 
No, not always. The Fifth Doctor travelled with Turlough solo for bit before he left. But yes, if it was just another guy it would just end up like Sherlock.
But I didn't necessarily mean JUST a bloke; and I'm not saying I immediately dislike the new companion. I just think that a male and female companion, neither of whom are into the Doctor or each other romantically might have been nice though. Along the lines of Barbara and Ian, Jamie and Victoria, Turlough and Tegan, or Jack and Rose.
(Actually I really don't like that way Jack has just vanished from the Whoniverse. I reckon he's a cracking character who could still offer a great deal to the show. For that matter, the fact that the Master is now Missy also means the repertoire of strong male villains has also been reduced. But I digress.)
Listen to the description of the "new companion":
Pearl Mackie play a character named Bill, who she describes as "cool, strong, sharp, a little bit vulnerable with a bit of geekiness thrown in."
like I said, I'm not writing her off, but I mean seriously? How fecking generic is that? How many Nu-Who companions could this description be applied to? It may as well be the same character regenerating.
I'd actually love to see a new alien companion, or at least a companion who isn't a present day human. I reckon they missed a trick not having a Time Lady stow away in that TARDIS the Doctor nicked from Gallifrey at the end of the last series. Or it could have been just a bog-standard Gallifreyan techie.
Look at it this way: the last recurring male character was Danny Pink. Danny. Pink.
