I think the reason low resource languages are prioritized is to compensate for the fact that AI research normally has a tendency to marginalize these languages.
Low resource in this context means that there are few resources available to train a neural network with, not that there are few speakers. Although many low resource languages have relatively few speakers, there are also ones with tens of millions of speakers.
The reason for emphasis is in my opinion twofold: 1) Allowing these people to use the fancy language technology in their own language is good in and of itself. 2) Training neural networks on fewer resources is more difficult than using more resources and therefore a fun and interesting challenge.