If a hermaphrodite animal (like slug, snail, etc) finds a partner they can mate immediately.
If another animal with "normal" reproduction (lets say a mouse) finds a partner they can only mate if they have the opposite gender.
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' hermaphrodites can reproduce more easily, but other organisms invest more in their offspring and also often have a harder time getting around to mating. It is perhaps easiest to address the question by countering it and asking why dioecy (2 sex systems/2 gonochoric types e.g. As you have pointed out there are obvious advantages to being a hermaphroditic species such as more chance of mating - more likely to provide an advantage at very low population densities where interactions are infrequent.
Therefore at higher population densities, when mating opportunities are not rare, the gonochoric individuals will have a higher fitness because they have more energy.
Monogamy is also analogous to rare encounters but true monogamy is rare (1 partner for life).
So it seems logical that the hermaphrodite way of reproduction is more successful than the "normal" way. The group of fish known as hamlets en.wikipedia.org/wiki/Hamlet_(fish) are simultaneous haermaphrodites, like snails.
But it is not, as far as I know all higher developed species are using the standard way of reproduction (male and female). Many other fish are sequential hermaphrodites, but that is probably outside the scope of your brief - its quality not quantity.