Computer USB ports generally provide less current compared with wall chargers. Just guessing, but with the hotspot on, the phone requires more current from the battery than the USB port you are using can provide.
I hotspot at clients for a day's work connected to ASUS ZenBook UX305 and the 6s is at 100% charge at end of business day when I depart. How old is you computer? USB 1.0, 2.0 or 3.0 ports?
Re: iphone 6s why does turning Personal Hotspot ON stop my phone charging from computer?
I could be mistaken, but I don't think USB 3 makes a difference in power provided unless the cable is also USB 3 which the Lightning cable (afaik) is not. I would have to agree though. Probably the fact that the USB port just simply can't provide enough current. USB 2 is limited to 250mA per port where as the wall charger for the phone is 1 A at least.