Crossbows use dex to hit, javelins use str to hit. Crossbows have 15/30 range, javelins 10/20 range.
A warden will probably start with +6 to hit with a javelin and +2 with a crossbow and the gap will grow.
Also a crossbow needs 2 hands to fire, a javelin 1 to throw so it's much more convenient...