The style you were using was a stock vbulletin style. It's the basic style that comes with the original vbulletin software.
Each style has individual coding differences, they draw on different imagery and have different embedded code to do certain things both functionally and decoratively.
Two main things necessitated the change:
One - we now have some paying sponsors / advertisers who expect to see their code displayed properly regardless of the skin a member may choose. The "stock skin" was producing errors when that code was inserted.
Two - and this was more a styling concern. New unregistered members see an unmodified skin and have an immediate perception that the admin doesn't care enough to invest the time and money in skin creation instead choosing to run with vbulletin's stock skin that reflects nothing of a site's personality.