Because commercial success is important. They make music as their job and want to sell CDs. It's not their hobby. And the music biz became very hard. When you don't have any success (or even only less success) anymore you loose your record deal. Look at Mariah Carey who sold way more CDs than Bj but when her last album wasn't that successfull she lost the record deal. The same for Michael Jackson.

Because of the critical success. It's always well known that things (not only albums but also theatre plays and stuff) that have negative reviews are the most successful. The critics don't like it but the people do. Why should they care about critics? They sold millions of albums even though they have negative critics. And that's what is important.