ToV is much harder than EN, it's a filler raid after a filler raid, its rewards are uninspiring for the difficulty even after the buff, and for Nighthold preparation guilds are likely better off farming M EN and 2/3 ToV rather than crack their teeth on a very hard boss that only offers loot 5 ilvls higher than the previous two bosses.
I doubt it's a real decline, just a lack of interest in that particular raid/boss.