Because I don't need the 4K aspect which is why I got the Pro over the Adv. Once I got it and started using it for Commercial Ops I realized that none of my clients can utilize 4K video. If I shoot in 4K I have to downsample it to 1080p. I do realize there are several advantages to shooting in 4K and then downsampling if needed but to date I've not "needed" it yet but keep in mind that most of my work is Realtors, Architects, Building Contractors, and local News Papers. UHD isn't needed and even the pictures I take I have to often times crank them down to fit within the file size requirements my clients use.
If I had gotten the P3A I could have used the extra money to buy more batteries and so far I've never said, "Dang I've got too many P3 batteries." LOL