Probably for a number of very good reasons. It's their firmware, for their machines, they want to control it and they want it to work the way they desire it to do so.
To release the source code allows their competitors access to their intellectual material, which they pay their engineers to write. It would also let people hack it as they saw fit, for good or bad, and those hacks would then be released out into the world and would possibly reflect badly on iBasso.
And I would be quite loathe to put such a firmware on my machine. Who would know what was in it? With a Mac I should probably be somewhat safe, but for PC users, who knows what might be lurking within?
I can wait a "few days" to get it from iBasso, all things considered.