When you have a tablet, you can do things like punch in what defense the other team just used to provide statistical analysis of what the next best play is, or what kind of defense to run if your opponent is doing X often.
None of this should be done on the sidelines. This should be done in the team's viewing booth where weather is kept outside. A good connection can be provided for the team's cadre of wonky strategists to use up in the booth, and they can confer without the distractions on the sidelines. Much of this can also be done half a continent away at the team's headquarters. The sidelines are not the place for a data entry team and a data analysis team to do their work.
Weather is also a complication. For much of the American football season, the weather will be a major factor in trying to use touch screen based technology.
One of Belichick's complaints, in particular, was that they could not get the series of overhead photographs of the previous play down to the sidelines quick enough via the tablets. A few years ago someone would print them up in the viewing booth and a runner would take copies of the photographs and deliver them to the sidelines. Sometimes a printer would be set up near the sidelines. The older methods were faster than the tablets, probably due to poor connectivity on the field.
Besides the speed of delivery, the tablets introduced a slew of usability issues that physical photographs did not have. The photographs would be viewable in bright sun, rain and in snow. It was also easier for multiple people on the sideline to view the same set of photos at once (no darkness when looking from acute angles). It was also easier to view multiple photographs at once, as opposed to one at a time using the tablet.
The tablets are a solution to a problem that did not exist and are acting like an anchor instead of a sail.