Complex sentence: The band had intentionally picked those which they deemed the more conventional and "straight-foward" to make up Showbiz, while leaving out the more experimental and progressive material to be included as B-Sides in the single releases.
Split into simpler sentences: The band had intentionally selected the songs which they deemed the more conventional and "straight-foward" to make up Showbiz. The more experimental material was left out of the album to be included as B-Sides in the single releases.