Why you don't accept bands which released 1 metal demo tape, and changed their style after release?

I think the archives does accept bands with one metal demo tape.

?

Because Morrigan has decided so, if the rest of the bands lengthy career is not metal.

If you want to suggest a change, you have to ask her first. There are no definitive written rules on it, I would not mind having such bands here personally, if their demo would contain 3 or more songs.
If the situation would be, that the band has two demos only, one metal and one non-metal, I would tend to accept it, same for bands with one metal and one non-metal single as their only releases.

The bands are judged individually, so you have to ask for a certain band - which one is it now? Opča Opasnost again?

No, I just ask, for every band. Opća Opasnost never released demo tape, so, they are excluded.