Well, if you want practical reason why Hobgoblin wouldn't take the Green Goblin stuff as is... well, he's an independent villain and wants to establish his own identity. He doesn't want to be known as an offshoot of Green Goblin. Plus, he's a fashion designer. He knows that the Green Goblin mask sucks. So he keeps the armor (but modifies it), adds a cloak for effect, changes the color scheme, and changes the mask.
Of course, the problem with an "organic" mask, is that it's sure to melt under high temperatures, which, as a supervillain who uses explosives, you have to be prepared for.