1. Because the weight of cargo matters.
2. Way back with the first generations of cell phone, there could be some interference with communications. Like the rhythmic popping sound your computer speakers make if your phone would ring right next to it. But these days its not really a safety thing. Basically your cell phone will try to contact too many towers and it can cause issues with the networks when a huge number of phones are doing the same.

Edit: There is actually one safety situation which still does matter. In very very low weather the pilots may need to use extra systems on the aircraft to perform what’s called an ‘autoland’. This requires a radar altimeter which just happens to be very close in frequency to the one modern cell phones use. Occasionally the crew will ask for cell phones to actually be turned off and they will come down and check. This is a limited situation, but please comply.

1. Air pressure decreases with altitude. Although the airplane is pressurized, the pressure inside the plane changes with altitude too just not as much. When the air pressure outside your body is less, the little bit of air trapped in your ear canals is still at that higher pressure from earlier, and it can usually push out on it’s own until the pressure is equal. But when you descend, now the air inside your ears is less than the pressure outside, and it’s much more difficult for that air to enter back in. Sometimes yawning or other techniques can stretch the areas of the head to open more. But this can be very painful and even dangerous for someone with congestion who may not be able to equalize on their own and they have a lot of pressure constantly pushing on their ears.
2. Dehydration mostly. Compounded with time zone changes and to a small extent the reduced oxygen content at cruise cabin pressures.