The answer to your question is cost. Maintaining two versions of the site is quite expensive; every color and GUI decision must be carefully verified against two layouts, essentially doubling the work. Most companies, including banks, forgo the second layout because the costs can quickly add up, and the same goes for large corporations.
Unfortunately, it's not feasible for me to support more than what I'm already managing. The only reason I continue to offer the light theme is that the users have not complained about it being "too bright." Otherwise, I would eliminate it and save money. We have huge budget constraints.