Because they didn't bother to do the work to make that relationship fly for fans. It was just...there.
I do sometimes wonder if DC and WB even know how fans view this incarnation of Power Girl? It seems to be mostly negative, with the view being that this character ISN'T Power Girl. They certainly don't seem to be doing any course correcting at all AFAIK. The art for PG is fine for the most part, but the writing, at least in her own series is way off.