Why SGI members fail to attain Buddhahood

"In our present day, the Lotus Sutra is of course supreme, as it was in the past. And yet, because the way of practicing it differs from age to age, in this era, even if one were to retire to the mountain forests and read and recite it, or live in the villages and expound its doctrines, or observe all the various precepts, or even burn one’s arms as an offering, that person would nevertheless fail to attain Buddhahood." -- Letter to the Lay Priest Ichonosawa

The way to practice the Lotus Sutra in this era in order to attain Buddhahood is by admonishing slanderers. SGI members, though they "live in the villages and expound its doctrines" practice interfaith. Failing to admonish the slanderers of the Lotus Sutra, they thus fail to attain Buddhahood.