In addition to the reasons given by other replies, “eligible voter count” is readily manipulated. If you create incentives to manipulate eligibility, you will see a lot of manipulation of eligibility.
There is no “official” count of eligible voters equivalent to the census. Voter rolls are generally maintained at the county level, with a wide range of resources, policies, procedures, technologies, and ethics applied to maintenance of the lists.
State legislators have some leeway in determining the rules for eligibility. The easiest item to manipulate is residency rules (great for disqualifying large numbers of college students), but legislators have a lot of other rules they could put in place that would favor certain groups. For example, strict rules about notifying voter registrars when you move would favor homeowners over renters.
Mostly, though, you just need to know that the sole reason the effort exists is to put even more power in the hands of groups that have higher turnout – the wealthier, the older, the more whitened.