This is addressed in the film's Visual Dictionary. Recognising that a siege is taking place, the First Order airlifts a bunker-buster to the battlefield. This Superlaser Siege Cannon is a highly vulnerable piece of kit while it's charging (and apparently has a line-of-sight range) so the goal is to keep it safe until it can be used at least once. This means landing outside the range of the Resistance's gun emplacements.
"To penetrate the Resistance defences, the First Order airlifts a massive superlaser siege cannon to Crait's surface. Towed by tug walkers,
the enormous 200-metre-long weapon is a piece of miniaturised Death Star technology. A fiery tracer beam creates a path along which a devastating pulse travels towards the target, detonating with terrifying impact. The cannon requires time to charge up between blasts, however, during which time it is vulnerable, necessitating escort walkers and air support."