There's a secondary, and frankly, equally important reason they never used it again, in addition to what
@Gehenna said - which is that as a whole, the DNC fucking
hated the 50-state strat. There's a few reasons they hated it.
First is the fact that the DNC relies on its shitty "only stronghold states matter" strategy because it's part of what kept the Uniparty functional. It helped keep the early progressives and oldschool dems who wanted to fix shit
out, and kept the establishment hacks
in. Conversely it also helped justify all those Blue Dog triangulators.
Second, it cost the DNC money to actually do, and if there's one thing the DNC hates more than its own constituents, it's having to spend more money, even if there's a payoff.
There's a reason that the second it was successful once, the DNC kicked Dean the fuck out. Gehenna's quite right that it was likely to only work once before being compensated for, but the DNC's hatred for it had a lot to do with why it got abandoned.