How can you not understand it, when it's been explained in this thread? You may or may not agree, but I find it bizarre that you don't understand.
It fugged up game balance and undermined the atmosphere (one of D1's greatest strengths) with low quality content. More is not always better.
Case and point, you say the balanced was fugged, but D1's balance was shit, because Sorcerer is objectively the best class by huge leaps and bounds, and some of the classes added in Hellfire narrowed the gap a little bit. Therefore, I'd argue that Hellfire is better balanced than vanilla. Come at me.
Some people prefer to mod the original D1 to get the best stuff from Hellfire without the stuff they don't like like, and that's the fairest point I've heard.
Undermined the atmosphere? Retarded opinion. The added areas are optional, some have stated they like them, some people like me don't have a strong opinion one way or another about the added areas. But nobody can argue that it takes away from the original content in any way.