Muramana is bad is because there aren't items to support it
Or that's part of the reason, at least.
With Seraph's you can pick up Lichbane or Rod of Ages to help you get the most out of its passive.
But for Mura? You've got Trinity (sort of) and that's it. If there were a Spellblade item less focused on Crit (
kthnx) and an AD item that built out of Catalyst then I suspect Muramana would be quite a bit more attractive.