To be honest. I had this discussion on comment on Youtube before. When I GM I tend to use Charisma if the players do not motivate why they should use Strength instead. The way I view Charisma is basically your general Force of Personality, the way you present yourself and how others view you. When you intimidate you want the one you intimidate to feel scared and threatened, which is why Charisma is being applied instead of Strength. If I want to for example threaten someone with my pure physical prowess then Strength is being applied. If you are looking for a intimidating presence you can just add a passive intimidation score which is 10 + Strength (Athletics), just like passive perception is 10 + Wisdom (perception). However to answer your question more directly. It seems that the Intimidation check is hard coded to be Charisma based, but you can manipulate that if you want. In the character sheet there is a cog that most likely can only be viewed by the GM. There is then a Skill Options section that allow you to switch if the skill is normal or expertise. You can also add a modifier to any skill. So if you want it to be Strength based, you can just add the difference between the characters Strength and Charisma as a bonus modifier to any intimidation check. It will say that the check is Charisma based, but it will essentially be Strength based for all mechanical reasons. The biggest downside is that when you level up Strength, you have to manually change the modifier to update the intimidation check. But it is at least less work then doing it with pen and paper.
