Because it’s none of their business how managers competing in several competitions choose to manage their squads and its none of their business which competitions managers choose to prioritise.
It would also be nigh on possible to police. A manager can simply say a player is “a little jaded” or not “at a hundred percent”, how can some suit sat in the Premier League HQ tell them differently with any sense of authority?
What if a manager is accused of weakening their side by not playing, for example, Ramsey and instead playing Elneny. Many might view that as a weaker side but the manager could easily justify it as being a tactical decision, playing a different sort of player to address a strength of the opposition. You could choose to play Iwobi over Mkhitaryan because Iwobi might be better suited against a certain fullback or team, others would argue that that’s weakening the side. I’m using Arsenal examples but you could literally pick out hundreds of examples from other sides too.
And if the Premier League get to make rules about having to play your strongest team then why can’t the bodies in charge of the League Cup, FA Cup, Champions League and Europa League rightfully do the same? Then you can end up in a situation in which managers are told that they have to play their strongest side in all 55 games or whatever game a season.
It’s just my opinion but such rules would not be appropriate, and even leaving aside whether the rules would be good or bad, they’d be totally unenforceable and completely down to interpretation. There’s no way such rules could be applied fairly across the board.