This is just definitely a side note, OP. This is my own personal experience for a situation similar, but not exact, to yours.
I consider myself a late bloomer in terms of appearance. All throughout middle school, and high school, I was unkempt and not considered pretty, to myself, or to others. When I started to become more interested in fashion and makeup, I found it confusing, and I never developed much self confidence for my outer appearance. I grew into my looks in college, but never truely believed myself to be as beautiful as some girls.
I had difficulties making friends, with either guys or girls, in high school and college. I had low self esteem growing up, so I would immediately get jealous of other girls who were more attractive than me, or who were more successful be it in friendships, relationships, school, or appearance. And I would gossip, and hate, sometimes, because I was under the notion that this person was better than me, and I felt inadequate. It was not healthy, but it was innate at that point, and all due to insecurity.
I felt like a lot of girls weren't interested in becoming my friends, either. It seemed like girls were constantly snooty towards me, or they'd overlook me and be clique-ish. When I went to college and joined a certain organization, someone started a rumor that I had plastic surgery. People talked behind my back, but this was only brought to my knowledge very few times. I wanted to tell myself that they were just haters, and in efforts to raise my self esteem, I told myself that every girl who gave me a "look", ignored me, or said bad things about me were jealous because I got some (not a lot, but some) attention from boys, and I had grown more into my looks. I had this constant "woe is me, nobody wants to be friends with me because all of these girls are jealous." attitude. But deep down, I really thought "woe is me, i'm not good/pretty enough so no girls want to talk to me." When I would be paranoid about situations, Nobody ever even suggested to me that these girls were jealous in the first place, I just tried to tell myself that to validate their actions.
But since then, I have taken a step back and realized that I was being completely and utterly self centered. A lot of the times, the girls I would think would hate me would actually never really talk about or comment on me much, and it was all in my head. I was completley just being paranoid. Of course this may not always be the case with everyone, but it certainly was for me. Once I realized that, everything became clear. I don't want to keep telling myself "girls are catty! i'm too good for them!" yet at the same time I refuse to keep telling myself "i'm not good enough to be friends with these girls." To me as of now, it is important to be happy and comfortable with who you are, regardless of what others think or say. Because in the end their opinions of you are irrelevent, what really matters is your opinion of yourself.
One thing that I learned is to block out everything that is negative. For whatever reason these girls aren't talking to you or are making snarky comments about you, it should be irrelevant, and it's useless to care about it. As long as you work towards being a good person who is true to herself, you will attract the right people and friends. And then, you will be surprised how quickly all of that negativity around you will just melt away. Focus on the good things, ignore the bad.
I am sure you have people who love you in your life, and that is much to be happy and grateful for! Still, to this day, I don't have that many friends but I still have a family that loves me, and that is a great gift I try to cherish while I can!
So, in the end, to answer your question, why do girls hate you? It really shouldn't matter why. If they are hostile towards you, they are, ok, go on your merry way. I know who I am, I worked hard to be where I am, you can say what you want. They will back off or get bored once they see that what they say doesn't effect you. Don't let them win at this game they play. In fact, don't even play in the first place. It's not worth your time.
In an ideal world, right? I know for a fact that this advice is hard to follow, especially for me, a girl who is trying to gain buckets of confidence to make up for all the times she had none. But when I feel down now, I always tell myself this to get me through the day. It is positive thinking and it really does help me cope with things!
Congrats on your promotion, and I hope this advice helped a little bit! I know I blabbed and more than likely got off topic, lol...but oh well. GL with things.