Because there's a lot of dumb shit wrapped up in Bruce/Barbara. If circumstances were different, people wouldn't give a shit. Part of it is Barbara hate in general, a sentiment initially fostered almost decades ago now when some dumb writer got a hold of her and began using her to sabotage Dick/Starfire. Some of it is more recent, when Barbara went from being a character who had her own arc and circumstances to being "the one true Batgirl, eternal youth hero." In a similar way to all the dumb shit with Peter Parker, really. But unlike Peter, this came with sidelining and character assassinating other Batgirls who had their own vocal fans.
Then there's all the actual Bruce/Barbara bullshit, which represents some of the worst excesses of Bruce Timm's side of the Batman franchise and introduces even more annoying melodrama in a set of characters already drowning in it. Most of the hate again fall on Barbara more than Bruce, but people aren't really fond of it all around and relate a variety of specific reasons why. It's just the totality of the package, most of it baggage from how Barbara has been handled and mishandled over the years, that causes problems. There's just too much nonsense around the character and her place in the Batfamily.