Because the 'O0' confusion is more common than the 'Ø0' confusion, probably.

I personally prefer having the zero glyph with a slash only on the inside (i.e. not protruding), or with a dot on the inside, but whatever.

its one of two or three accepted solutions to the 0O problem

As an english speaking American, learning danish, I agree it is a little confusing, the zero versus Ø problem. but it is also confusing to see 0 and O written when the zero has no slash. I think maybe our zero needs a less ambiguous distinguishing feature. I can easily discern 0 and Ø but I think it may be that I am so used to seeing 0 with a slash.