It could be that these "new" bands have a back-log of great songs that they wrote in the 15 years before they were discovered! So they had a lot of choice and good songs to record on their first album. Then they are pressured by their record company to record another one quick quick quick, if the first one was successful. As a result they haven't had time to write proper brilliant songs, so they churn them out, and the songs are just not as good quality as the first ones.