Why didn't Ovid include the Heroic Age?
Subsequent authors were always changing up the content of their predecessors to keep things fresh. (Simply repeating Hesiod wouldn't get an audience excited. Contradicting previous authors or adding new twists to the old stories sells books, or scrolls, as the case may be.)
On the other hand, Ovid may have simply been attracted to the idea of comparing each age to a metal, and decided that the Heroic Age and Bronze Age could be merged.
Ovid was writing poetry, thus keeping the metal association strengthens the metaphor. It also squares nicely with his mention of the creation of the Four Seasons.
(You can read an old fashioned, poetic translation in English here.)
It may also have to do with the improvement Hesiod references during his Heroic Age. Ovid's four ages seem to reflect a steady, downward progression as civilization advances, which he may, again, have felt strengthened his poem.
Hesiod was himself a poet, but his life and concerns were quite different from the much later P. Ovidius Naso. For Hesiod, content may said to preempt form (the tension over form vs. content is central to poetic composition.)
Hesiod clearly wants to make the case that the heroes of the Iliad and their generations were superior to the debased times he himself lived in. Fidelity to the metal theme was clearly not as important to him as his moral message.
Where Ovid's primary intent is aesthetic, Hesiod's primary intent is instructive.
[PS-This is just a quick overview--I may have more info once I get a chance to do a little research.]