Part of the problem is that it's hard to get companies to "donate" money. If they can purchase something like "support" or something else tangible, or "sponsorship" out of their advertising budgets, there may be more opportunities of gaining funding.
Thank you for donating though! Django gets a lot of their money from regular developers' pockets. I, personally, feel that bigger companies using Django to generate lots of revenue should be contributing a greater portion to support the stack that supports them.
[0] https://www.djangoproject.com/weblog/2015/nov/23/introducing...