That area is exceedingly rough terrain, especially in winter and early spring. Everything is hilly and uneven, and the land is full of crevices and unexpected dips. Lots of boggy bits, lots of streams that look safe to ford on foot but are unexpectedly deep or fast moving, and terrain that was previously stable might have become unstable after winter storms. (That last one can be insanely dangerous, one minute you're walking along a hillside path, it gets muddier and muddier...the next the ground just gives out from under you and you're sliding, unable to stop yourself as the mud and rocks tumble you down a slope with them...and it's up to gravity and god as to whether you'll be able to stop safely, or whether you'll fall off the edge of a cliff)
Plus, it was in March he went missing, when the days are still short, the temperature dips quickly and weather is exceedingly changeable. Quick moving mists and fogs rolling in can disorientate even an experienced hiker and send them off track.