Developing an official Instagram app for the iPad would indeed be a lot of work, even with an existing iPhone app and website to work from.
Instagram CEO Adam Mosseri recently stated that we don’t have an official iPad Instagram app because the company can’t devote the resources to it. This isn’t a hand-waving excuse, either—it turns out app development is quite complicated.
Even something as seemingly straightforward as a port would still require an awful lot of work. It’s way more complicated and involved than the average user might expect.