What evidence is there from scripture for why Daniel did not eat the King's food? Reading through Daniel brings up many questions.  One question that is brought to mind is "what evidence does the scripture provide to show why Daniel would refuse the King's food?"  I have read many opinion answers in a previous post: Why did Daniel consider the allotment of food and wine to be wrong for him to eat?

I am interested in specifically scriptural based answers.  I find in the text that Daniel and his friends were tested for 10 days and and at the end of the test "their features appeared better and fatter in flesh than all the young men who ate the portion of the king's delicacies."(Daniel 1:15)  Is there any more to it than that it was a more healthy option?
There are two primary, though related reasons for why Daniel would not eat the king's food and drink.

Daniel was an Israelite (Dan 1:3) of the tribe of Judah (Dan 1:6).  He would be bound by the dietary laws regarding clean and unclean foods (see commands for example at Lev 11:46-47).  As well, there was preparation and cooking instructions to be followed.  Daniel had no control over these things as regards the food, but only the vegetables and water.

The other reason is that most likely the king's food would have been offered to idols before eating.  This too was not permitted.


  It was the custom to throw a small part of the viands and wine upon the earth, as an initiatory offering to the gods, so as to consecrate to them the whole entertainment (compare Deu 32:38 ). 
  -Fausset-


So, the evidence from scripture about why Daniel will not eat basically boils down to it was against their Covenant.