It's the strain on the fea, the soul, that is the main reason. [ Maglor explains ]

Quite a bit of both parents go into the bearing of the child. Father was very worried about Mother, so there's almost eighty between Maedhros and I - they waited until he was almost to his full maturity and was no longer drawing on the bond between them. After that, the gap shrinks somewhat, to around fifty years, sometimes more, sometimes less. But many parents will wait for the first child to reach their full century before the next, and many never have more than one. Seven is unheard of, although even my cousins come from large families of four and five, respectively.

I know the differences for mortals are less acute, though.