I assume it is disallowed because spammers would bombard the platform (even more than they do now) if every photo could contain a link at the bottom. Limiting links is one of the best ways to prevent spam from overtaking a platform. I’ve been a Community Manager for a while now, so my professional experience with spam in its many horrendous forms is my basis for that belief. It’s always struck me as a smart move on Instagram’s part to be staunchly against links in photo descriptions, despite what must be an outright onslaught of requests to change this.
However, I’m now positioned within a Marketing team in my current job, where I get to see just how maddening this limitation can be for content publishers who use Instagram for revenue generation.