Sakanah

Personally, I don't follow this one....

Orthodox Jews avoid eating fish and meat together because the rabbis treated the combination as a matter of sakanah, danger, not as the same category as basar b’chalav, meat and milk.

The root source is the Gemara, Pesachim 76b, where fish cooked or roasted with meat is discussed as dangerous. The concern given there is rei’ach and davar acher, which Rashi explains as a danger connected to tzara’at, a skin affliction.

This is later codified in halakhah. Shulchan Aruch, Yoreh De’ah 116:2 says:

One must be careful not to eat meat and fish together, because it is dangerous.

The Shulchan Aruch, Orach Chaim 173:2 also treats separation between fish and meat as required because of danger. That is why many Orthodox Jews will eat fish first, then cleanse the palate before meat, often by eating or drinking something in between. Unlike meat and dairy, there is usually no six-hour wait, because this is not a Torah prohibition of mixture. It is a rabbinic safety concern.

A key principle is חמירא סכנתא מאיסורא, danger is treated more strictly than ordinary prohibition; see Chullin 10a. So even though modern people may not know the medical mechanism intended by the sages, halakhic Jews preserve the rule because it entered normative Jewish law.

So the short answer: fish and meat are not forbidden because they are ritually incompatible like milk and meat. They are avoided because the Talmud and later halakhic authorities classify the combination as dangerous.