San Diego gave him no chance to succeed. They lost Tony Martin before the draft, then traded Eric Metcalf and their 2nd round pick, plus their ‘99 1st rounder to Arizona to move up a spot and get Leaf. It left the Chargers with no real receiving threats, and Natrone Means was their RB - well past his prime - so they weren’t a great running threat either. No QB was going to have much success there in 98-99. After the way Stan Humphries got beat up in ‘97, it was pretty clear things were bad in San Diego.
they needed a QB who was strong enough to lose. Instead, they took one who had glaring maturity issues, fired the coach mid-season, and did basically nothing to improve the offense until after the 2000 season (when they dealt the #1 pick and turned it into LT, then got Drew Brees in round 2). By then they had given up on Leaf.