Because everyone’s schedule differs and with more than one daily Mass available it allows a chance for everyone to attend.
For mothers like me who work fulltime & have children from infant to teen - attending Mass in the AM is impossible the vast majority of the time, but I’m usually able to attend the 7:30 PM Masses and still manage to attend to all of my work related & household related responsibilities as well as meeting the needs of my children. I don’t know about anybody else, but I need the Eucharist to make it through!
And then what about nurses & doctors who must work a Sunday morning shift…they’re work is a necessity for the lives of others who depend on them, but if Sunday morning is the only time Mass is offered, then they have no hope to receive the Eucharist

sad!