If you want an official answer from Purism then you would need to contact support directly.
Everyone who has commented so far is not commenting on behalf of Purism, as far as I know, and so is only sharing experience or opinion.
The Librem 15 is 4K UHD, so I assume that the larger laptop does not suit you??
If you have normal vision then it would be overkill. 2560x1440 would be about right for the scenario of laptop use case and normal vision and 13" diagonal and 16:9 aspect ratio.
If you have better than normal vision then you could justify 4K UHD.
This topic has been discussed in the context of the Librem 5 phone.
Re: librem-5-smartphone-final-specs-announced - #176 by Jt0 and surrounding posts.
I’m using a 4K UHD monitor on a non-laptop use case with one of my computers and there are some software issues at the moment i.e. some software just doesn’t expect the pixels to be that small. So in addition to the economics, commented on below, there is a software maturity issue.
Another consideration is that Intel integrated graphics that supports 4K UHD @ 60 Hz is an issue (e.g. the CPUs that are in the current models of Librem laptops don’t). So in concert with upgrading the screen, they would be looking at upgrading the CPU, or compromising on the frequency. (They could go with a dedicated graphics card but for pure open source that is at best a world of pain and at worst a “no go” area.)
That would be my guess too.
Eventually 4K UHD will become commodity and 13" laptops will use that resolution ‘just because’, I would guess.