There were two main reasons (military and economic)
Czechoslovakian defenses were running through natural barriers as were the mountains located in Sudetenland. The Krkonose mountains or Jizera were deeply forested and fortified with lines of the man-made defenses that were build to match the most advanced military strategies of the 1930's. The attack on Czechoslovakia from the north as was usually traditional line of approach would drag German army into prolonged siege of the fortified mountain ranges. Last thing Hitler wanted to have another Verdun in the society that was still lukewarm toward total war. And the German army for example lacked equipment to deal with these defenses. Their tanks or heavy transport trucks were still primitive and often not better than Czech or French and improvement of the armament was just on the way.
Second reason was economic. Czechoslovakia was economically depended on the Sudetenland for two sources iron and coal. The majority of the steel, iron, and related industries were there. The coal provided energy for the rest of the country, plus the largest petrochemical plant in Central Europe was in Sudetenland, just outside of city of Most. The rest of the industries in Sudetenland had a limited needs for the Czech defenses, since it was based for export and production of consumer goods like was ceramic, glass, textile, furniture, and paper. The largest defense plants were inside of the country (Pilsen, Uh. Brod, Brno), not in Sudetenland.
Hitlers ambition to take Sudetenland paid off. Czechoslovakia was crippled, since Germany took the entire mountain ranges. What was left was indefeasible land, where Germany was for example only six miles from second largest city, Brno, 1 mile from its third, Ostrava, or 8 miles from Pilsen. Interior of the Czech Lands is rather flat and it was much easier to taker over without fight. Secondary, Germany took entire military equipment there that was used to wage a war and they did not pay for it. Every third tank invading Poland, France or Denmark was either made in Czech Land or used from the inventory left behind. Plus, mountain defenses of the Sudetenland were based on the French model, so German got pretty much diagram and plans for taking the France. And all this without firing one shot.