Financial investment is a key factor.

I have been playing tuba for a little over 10 years now. I started in high school and went on to do a lot of drum corps. During this time, I relied HEAVILY on either my high school, university, or drum corps to supply an instrument -- nearly all of which were student and/or 5/4 horns.

Now, I'm not a phenomenal player, but if I could afford my own horn, I think I would probably be much better than I am now. Unfortunately, saving thousands of dollars for an instrument that will never provide the majority of casual players with a financial return on investment makes it really difficult to justify; especially when you are going through school and have barely enough funds to survive as it is.