It's an aquihire - they get the core team on staff. They can influence the open-source project roadmap, and get priority support for their integrations.
Offering a great developer experience (DX) when building e-commerce sites is core to their strategy. DX was a key reason why they beat Squarespace in the early e-commerce days: developers preferred building Shopify sites over Squarespace sites.
It's in Shopify's best interest to maintain that developer love and Remix can help with that. Here are two hypothetical situations to highlight that point:
1. If someone else, say, Swell [1], had better DX than Shopify, we would see Shopify start to lose market share to Swell for the segment of the market where developers can influence the tooling decision (i.e. agencies).
2. If in two years from now, everyone is using Next.js to build e-commerce sites, that would put Vercel (the company behind Next.js) in a good position to promote/partner with other e-commerce providers or build their own e-commerce solution and compete with Shopify.
The developer market segment is important and DX is key there.
It's pretty decent, passive marketing, too. One could argue a similar effect would be achieved by a fork, but I don't think it would ring as true to SWEs.
I think about Google a lot more due to how prolific Material Design is throughout my web experience, I think about Meta a lot more due to how prolific React is throughout my web experience, etc.
Rewinding earlier, what is the case for VC investment in an open source framework? Usually it's some variation of proprietary extensions ("open core"), enterprise support, and/or hosting. Docker, Red Hat, MongoDB are prominent examples.
So that could also be the rationale for Shopify acquiring it. Alternatively, they could've just wanted to acquihire an excellent team.