For a closed system, those kinds of open connections are deeply dangerous. If anyone on Instagram can just link to any old store on the web, how can Instagram — meaning Facebook, Instagram’s increasingly-overbearing owner — tightly control commerce on its platform? If Instagram users could post links willy-nilly, they might even be able to connect directly to their users, getting their email addresses or finding other ways to communicate with them. Links represent a threat to closed systems.
Precarity and Scarcity
Here's the thing, though: people like links. So closed systems have to present a pressure release valve. Hashtags are a great way out. They use the semiotics of links (early versions of hashtags on social platforms were really barely more than automated links to a search for a particular term) but are also constrained by the platforms they live on. A hashtag is easier to gather into a database, to harvest, to monetize. It’s much easier, sure, but it also doesn’t have all the messiness of a real link. Instagram doesn’t have to worry that clicking on its hashtags will accidentally lead people to Twitter, or vice versa.
And the ultimate triumph of being anti-web is to make links scarce. The smallest possible number of links a platform could allow is zero, so Instagram gets as close to that theoretical limit as possible, and gives you… one. You can have one link. Aren’t you grateful? One!