Linux

Everyone will claim it is the hardware, but we can see from cheap phones that a majority of people actually get outside of the US that it doesn't matter as much.

It was never a complete phone after 5 years. It never had the software to actually use it as a daily driver. Calling still "doesn't work all the time" according to users and similar with texting. If your phone literally can't be trusted to make a simple call and receive a text out of the box, then it won't be bought to be used as a normal phone. That's as simple as it gets.

It has just been relegated to being a fun side experimental phone for enthusiasts, but you can't have a company-carrying product like that because the consumer base is too small to fund the software development.

They also specifically say

While in the future the PinePhone Pro will be able to serve as your daily-driver smartphone, at present the PinePhone Pro should be considered a development platform.

On the store, which further discourages consumers.

Building a smartphone OS and all the features needed is an extremely expensive task, so it is completely understandable that it has gone at a snails pace.