I read that Vic Armstrong was tied into doing the new Mummy flick, so he was unavailable for the new Indy film and they went with someone else.
from Wikipedia:Victor Monroe Armstrong (born October 5, 1946) is a BAFTA winning British film director and stunt double -- the world's most prolific according to the Guinness Book of Records. The strapping, 6-foot Armstrong doubled for 6'1" Harrison Ford in the first three Indiana Jones films, 6'2" Timothy Dalton for Flash Gordon and 6'4" Christopher Reeve in Superman II.
Reportedly, Armstrong looked so much like Harrison Ford that the crew members on the films were constantly mistaking him for Ford. This proved useful when Ford injured his back and had to sit out for filming crucial action sequences in Indiana Jones and the Temple of Doom and Armstrong filled in for him. Indeed the stunt where he jumps from a horse onto a German tank in Indiana Jones and the Last Crusade was voted one of the Top Ten film stunts of all time by a panel of experts and Sky Movies viewers in the UK in 2002. On a private photograph taken on the film set, Ford wrote to Armstrong, "If you learn to talk, I'm in deep trouble!"[1] Armstrong was unable to work on the fourth Indiana Jones film, Indiana Jones and the Kingdom of the Crystal Skull due to commitments to The Mummy: Tomb of the Dragon Emperor. However, he had discussed possible action sequences with Steven Spielberg during production of War of the Worlds.[2]