Maybe a trust issue? e.g. Mozilla did not want the Debian team the ability to create some strangely modified version of Firefox that would be mistakenly connected to Mozilla.

Back then perhaps there was not enough context to know that Debian could be trusted.

The interesting thing is that the pain actually did go both ways. The lack of Firefox and the need for Iceweasel actually confused me as a young developer, and put me off Linux for a long time, because it made it feel like a cheap knock-off. I was unaware that they were otherwise identical.