Because for us is considered a sort of "crime against italian food and culinary tradition", because even if ketchup has some affinity with tomato sauce, it hasn't the same taste. Ketchup is way too sugary and many italians don't like his taste with the pasta (maybe someone could like it but they'll never tell you). Also all of this hate against "pasta with ketchup" has been a little overexaggerated 😌 I mean if you like it you can eat pasta&ketchup but this is not an italian traditional meal
You can still eat it if you want to. Of course, you won't find any restaurants serving ketchup on pasta here and I'm pretty sure there aren't many restaurants around the world which serve it as well. I mean, why would you when you can use real tomato sauce which is healthier and tastier? However, "de gustibus" (in matter of taste there can be no dispute) so eat whatever you like 😄
Cause it's like a murder. We aren't so proud of our country and traditions, but food is completely another thing: try to change traditional dishes adding the "wrong" ingredient and you'll be hated by every single Italian!