Prepositions – Why is ‘On’ Used for Weekdays instead of ‘In’?

From the context, it seems likely that omitting "on" was a journalistic decision.

The Associated Press Stye Book, used by journalists and others when writing public press releases, considers "on" before a day/date to be unnecessary, and therefore a waste of space in print. Omitting it is a habit that's a throwback to pre-internet days when paper size mattered :)

A journalist following AP Style might write:

"The event happened here Tuesday, in response to the other event that occurred June 10."

Both the preposition "on" and the "th" from "10th" are omitted to save space.

In spoken conversation or non-journalistic writing, people typically include "on," even though it's not required for meaning.

I am afraid that, as with most "why" question about language, the whole of the answer is:

Because that's how English is.

There is no other answer.