It seems like there are 2 reasons a musician will make a solo album rather than making the same album with the band they originated from.
1. The band is done, finished, over. The only way the artists can make another album is to do it solo, or start/join another band.
or
2. The material the artist want's to record is a departure from what the band typically records.
Number 2, seems to be the most common reason.
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"I used to be indecisive.......... Now I'm not sure."