You know Bellingham joined Dortmund in 2020, not 2021 right? Utd hadn't qualified for the CL back to back, they'd finished 6th and 3rd, having been garbage for most of the 19/20 season too. I'd say joining a pretty rubbish team with no long term plan, with a PE teacher as manager, who nobody believed would be there for the long term is plenty of reason why a promising youngster with loads of offers from around Europe decided not to join Utd.
It was widely reported that Utd were very keen to sign him and while you might not want to believe it Adam, he chose to join Dortmund over Utd. As I said, Klopp pretty much confirmed that Liverpool wanted him too, who were 100x more attractive than Utd at that time, but that he was set on joining Dortmund.