Because "ketchup" was traditionally made with tomatoes 🍅. When "tomato ketchup" was invented, it was like snake oil. Marketed as a medicine.
Tomato ketchup just became synonymous with the word ketchup...
Now... Is it: "Ketchup" or "Catsup?"
I guess there is a difference!
https://www.diffen.com/difference/Catsup_vs_Ketchup#:~:text=Catsup may be made of,be more spicy than Ketchup.
"The basic ingredients in modern ketchup are tomatoes, vinegar, sugar, salt, allspice, cloves, and cinnamon. Onions, celery, and other vegetables are frequent additions. Catsup may be made of tomatoes, onions, cayenne, sugar, white vinegar, cloves, cinnamon, celery seed and salt. So the two do not differ much in their ingredients. But different manufacturers may use different ingredients for the two. Sometimes Catsup may be more spicy than Ketchup."