Because he's got an eye for both. Tyrion is adept at both the bigger picture and the ground maneuvers (to use video game RTS terms, he's great at the macro and the micro). There's a reason why he's undefeated at Cyvasse, after all.

Like I said, lack of access to information is why they lost these battles, not from lack of strategy. Jaime has always been a formidable general in his own right and remember that Cersei still is Queen Regent... it's reasonable to assume Qyburn has ravens in allied lands apprising them of any of Dany's troop movements.