Because OP said he already listens to them.

My guess was that the first couple of singles advertised the album initially, and then subsequent single releases were to keep advertising the album after the initial spike in sales - keep the buzz going.

When radio, TV, and charts were more important for promotion, it kept tracks from an album circulating publicly for at least a few months. This was always my interpretation anyway.

Came here to say Tigercub. Two very strong albums with an awesome range of influences.