Please Please Me was The Beatles' first album, so I'd imagine they were experimenting with the billing order before settling on the catchier sounding Lennon and McCartney. It's fairly well-known these days that many "Lennon & McCartney" compositions were not really co-writes at all, but were written by one of them, with the other making at most a minor contribution. I guess they just kept the joint writing credit for simplicity's sake.