why band gaps occur according to bloch theory?
Band Gaps do not occur according to a theory. It is the band structure (that has a gap only for semiconductors and insulators) that appears any time atoms are arranged in a periodic fashion to form a lattice. Bloch theory only says that for a definite K value, an electron that is in a band, has to have a precise energy depending on the "shape" of the band. This is similar to what happens when an electron is confined to a potential well, but in the case of a lattice, you have a (infinite) number of wells.
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