Money. Pure and simple.
If mobile and Switch was a priority they could throw a few more developers at it, that would cost more money. These porting companies work on other games at the same time which means varying priorities. They will tend to focus on where the money is coming from because it is a business and not a labour of love. Games like Stardew Valley, Undertale, Spelunky, Cave Story, Minecraft were all developed by one person. One person with passion. You cannot expect the same passion from developers working on a port. They're a bit like mercenaries in that respect.