Pine64 has other CPUs they can work with, but they are not well supported yet. I wouldn't be surprised to see another phone rev with a much better CPU in the future once the Quarts-64 line starts working well. Right now the people who can do that work already have plenty of hardware to work with, while people who can do phone work really want a somewhat faster CPU now.
MediaTek is horribly hostile to free software and would be a terrible fit in something like this. MediaTek is the reason the Gemini PDA couldn't do what the PinePhone is doing despite making some similar claims.
As for not waiting for a newer Rockchip part, it sounds like they are making it clear this is not a second generation PinePhone, just a more powerful version of what we already had. There will likely be a proper second generation with more major changes later on.