It’s matter of potential: can Ana save you, if you got bad luck(too much damage)? Yes, in many cases, they even had to nerf nanoboost from 300 hp restored down to 150. If she failed to, it’s mistake of Ana player, or unlucky player got too deep in trouble.
Can Mercy do same? Not really: if damage exceeds 50 hps(and “too much” damage usually is 100+) you are doomed, and she may not be able to resurrect you either, since it happened in unsafe location. Even if it’s good Mercy.
Players would rather have healer, that isn’t very mobile, but has decent chance of preventing death, than one, that can go anywhere, but with little success.
Nanoboost can and often is used as defensive ultimate. You got tank, that is about to go down, that prevents enemy from advancing? Give him nanoboost, and maintaining frontline becomes much easier for both of you. And it keeps your whole team, that is behind mentioned tank, out of harm’s way, when they would otherwise get erased as soon as tank dies.
Return 60 hps, or instant rez. Having utility with so many things, that can go wrong in the process, is liability in many cases. Too many times it’s better to not bother with resurrects, since alive Mercy is more useful, than dead one.