Youth in the music scene is like the kid who can't get onto a rollar coaster because s/he's too short.
Age, like height, is something outside one's control, and used as a measure of determining what is and what is not allowable. So what happens to kids who are defined as too short? There are kiddy coasters! This only serves to separate this population from everybody else-- they can either not go on a rollar coaster, or go on this kid version, either way they are denied access to a real rollar coaster. Similarly, for those under 21, there are occasional 18+ or all ages events. However, since most people are not under 21, this other-ized crowd is shut out from access to music.
Height is a safety issue because rollar coasters were not designed to accomedate people of all sizes. Why not? Because it is expensive; higher costs = less profits. Similarly, why arn't there more not-exclusive to 21+ shows? It's expensive and less profits. Most profits come from the bar, and since those under 21 are not allowed to purchase alcohol in this country, the more under-21-ers allowed in, the less you make in alcohol profit. Also, with the laws surrounding "all=ages events," at least in Seattle, it requires additional measures, like large insurance policy and extra security. These extra expenses decrease profits.
Summary: It is economically disincentive for all-ages music.