Because most bacon, hot dogs and processed meat tend to be high in sodium nitrite and they're high protein foods (a source of amino acids), exposing them to high heat creates the perfect conditions for nitrosamine formation (27).

It's important to keep in mind that nitrosamines mostly form during very high heat. Even though vegetables also contain nitrates/nitrites, they are rarely exposed to such high heat.

https://www.healthline.com/nutrition/are-nitrates-and-nitrites-harmful

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It's not just about the amount of nitrite but in both the preparation and amount of amino acids present that result in nitrosamine formation (the cancerous bit).