I think you're misrepresenting Instagram. Instagram is a personal PR platform where images are the primary vehicle. In that sense Instagram is closer to Twitter (text-based PR dissemination) than Unsplash.

Unsplash mostly served two groups:

1: people who uploaded as lead generation for professional photography, or some other product or service

2: people who just thought it was cool when someone used one of their photos

The only difference between it and Instagram is Unsplash didn't allow text images or videos.

I guess because this is Hacker News, you're technically correct. From an engineering perspective, you're right that the two platforms share many similarities. But from a business perspective they're completely different. Instagram is where normal people go to show off their life and absorb the lives of others via images. Unsplash is for photographers and consumers of stock photography. As a result of these differences, they have completely different business models, user experiences, and growth trajectories.