A reason is maybe they are not used to negativity at all, even the slightest amount. Others you can't really explain why they are sensitive, it's just how they are. You were just being honest and I don't think you deserved to be blocked. If I were in that girl's place, I wouldn't take any offense from that, I would consider it learning from spelling & grammar mistakes.
There are definitely a lot of sensitive people on here, but I've also found in my case that I don't word things properly. I made an opinion and people definitely took it a different way than I had meant it. I should have explained better, but even after I had explained it people were still against the opinion. So I just try to be very careful on how I say things now.