There are many reasons SGI members fail to attain Buddhahood. Here is one

"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

Nichiren goes on to say that the way to practice the Lotus Sutra in this era is by forcefully admonishing slanderers. SGI members "live in the villages and expound its doctrines" but since they practice interfaith, failing to admonish slanderers of the Lotus Sutra, they fail to attain Buddhahood.