It is easier to use the programs which are already available on the desktop due to the fact that GNOME (and probably KDE) already support touch screens. The usage of a tablet only slightly shifts from using a desktop. So it is a more gradual change.
So by rolling out tablets first, development time could be spent on smoothing out hardware acceleration, supporting cameras, responsive UI, optimizing touch, etc. And then we could tackle phones.
For me as a PinePhone owner, the biggest problem is not apps but overall speed. The OS feels very sluggish. And although I know that it is really old hardware, doing basic things should be much faster. This holds me back from really using it. The other big problem is battery consumption. Any distro I tried drains the battery so fast that I always have to shut off the phone when I am not using it. Even if I don't do anything. On a fresh Manjaro+KDE, I can see the battery drop by 10% after picking it up and using the browser for a few minutes.
And wrt to linux tablets: The PineTab[0] is sadly having really bad resolution (one thing that is excellent on the PinePhone btw.) and it is too heavy. It is a far less attractive package compared to the PinePhone which has "pretty ok" specs througout. So I am really excited for things like the cutiepie[1] or the jingos tablet[2] (if it is not a scam). Although the latter will come with a custom linux kernel which kinda makes the advantage of a (mainline) linux tablet worthless since one still depends on some people maintaining some old linux fork.
[0]: https://www.pine64.org/pinetab/ [1]: https://cutiepi.io/ [2]: https://en.jingos.com/jingpad-a1/