For an authoritative answer, you would have to ask Planet themselves. The gist of my limited understanding is that fancy parts from, say, Qualcomm, are really expensive, if at all available, lest you can buy them in vast quantities, which Planet can't.
Someone making millions of phones, can buy each part a lot cheaper and have a way lower manufacturing cost, per device, than someone making a few thousand. Lest their products were to become silly expensive, Planet must thus try to get the best out of simpler parts and simpler manufacturing than the bigger brands. Also, when developing a phone, you can't just buy any SoC (System on Chip, containing, among other things, the CPU) you'd like. You must license the intellectual property (patents and stuff) involved, before you can even, legally, buy the chips. Licensing alone could probably eat Planets entire budget, so instead they are working with an ODM (Original Device Manufacturer, basically a phone factory) that already has the required licences.
An ODM having licenses for chips that are quite expensive, lest bought in big volumes, is likely going to be geared for big volumes and, thus, probably not interested in, or perhaps even capable of, making small production runs. So, the choices of a small vendor like Planet (with roughly a 0.001% market share) seems, to the best of my understanding, quite limited. They had to find an ODM they could afford, who could make small enough batches, cheaply enough to, at all, make their original Gemini possible.
Planet chose to work with East Aeon. We have been told they are only licensed to use MediaTek, so at least until Planet can find, afford, and wants to work with, another ODM, MediaTek it must be.
