Because "this is our cheese" would translate into "Den her er vores ost".
"Det er vores ost" translates into either "It's our cheese" or "That's our cheese"
If someone asked you what was laying on the floor in English, you wouldn't say "this is my shoe", as though you were introducing your friend to it. You'd just say "It's my shoe" (Det er min sko). Same holds true in Danish.
"It is" always = "det er". It doesn't conjugate to a noun. E.g.: "it is a man" = "det er en mand" and "it is a child" = "det er et barn". If you say "that noun" then you conjugate it. E.g.: "that man is blonde" = "den mand er blond" and "that child is young" = "det barn er ungt"
It is because the three make out a language continuum and as such you could clame they are all just dialects of the same language. There is a saying "A language is a dialect with a navy and an army"
PS: As a Dane I really don't wan't to imply that Norweegian is the same language as Danish though... Because we kind of did something bad...