There are several reasons. Johnson could ID the talent, Jones could make the deal. Jimmy wasn't a deal maker. After the Herschel Walker deal, other teams were afraid to deal with him. Several of those draft choices were conditional. The Vikings sent several players to Dallas, and if the players didn't start, etc, those got converted to draft choices. Johnson sat all of them, and never gave them a chance to start, cause he always intended to turn them into draft choices.
Also, Jimmy got pretty lucky with the Cowboys with some of his deals. Michael Irvin was already with the team. Troy Aikman was the consensus #1 pick his first year, and the Cowboys had the #1 pick. Johnson picked up his old Miami QB, Steve Walsh, in the supplemental draft. That turned into the #1 pick the next year, so Jimmy didn't realize what he was getting in Aikman, cause he burned the #1 pick on a journeyman QB.
The Cowboys would have taken Blair Thomas over Emmitt Smith in the 1990 draft, but the Jets grabbed him. In fairness, Thomas had a higher draft grade than Smith, so a lot of teams would have done that, but that's what I mean by some of those player acquisitions just turned out well in Dallas.
Trivia, four of the first five choices in the 1989 draft were Troy Aikman, Barry Sanders, Derrick Thomas and Deion Sanders. When was the last time there was a draft that loaded?