Amy wanted marriage and kids and Umar always insisted he wasn’t ready. That’s why they split up as Amy is older and wanted that family unit which Umar refused to give to her. Plus he treated Amy completely different. Whatever Nada has done she has certainly changed him.
I honestly think it’s because she looks like a ‘Trophy wife’ and is also Muslim which makes it easier for him to marry, as he’s shown he practices his faith. Although his brother married Charlotte who is non Muslim so I could be wrong?