[QUOTE=Dinsdale]
Here’s one that numerous folks have tried to explain to me but it just hasn’t clicked. Why is music for different instruments written in different keys? Why, if every instrument in a concert band played the note denoted by the lowest line on the treble clef, would they not all be playing E?
[/QUOTE]
Accident of history. Sometimes, when things get standardised, they don’t standardise in an obviously-logical way.
However, there was never a pressing need for standarising on non-transposing systems, because once you’re familiar with what we have, it becomes second nature. And there are other less obvious factors which do have some logic within what is used today - bass clarinets have already been mentioned, where the transposition of a ninth makes it easier for a player to switch between parts. Likewise Eb and A clarinets, and the entire saxophone family also uses transposition so that all parts are written using the same treble-clef range.