The short answer as to why you should not record a podcast from your phone is because of potential audio quality issues. Your phone has limited memory, power and audio capabilities that cannot compare to a more professional recording setup.
Using a cell phone microphone to record your podcast will likely result in a low quality recording. There are many critical aspects to consider with audio recording such as the type of mic, the environment and the settings you’re using. When you’re using a cellphone mic, it is impossible to control all of these details, which will lead to poor sound quality and an inferior final product.
Also, a phone microphone picks up a lot of background noise depending on its location (e.g. a pocket), which will affect the quality of your podcast. Portable recorders, external microphones, and acoustic sound booths are much better for recording a podcast than your phone.
Finally, a phone will use up power quickly.