Warlocks that focus on Elderich Blast want two things: to cast Eldritch blast every round, and have a consistent use of their bonus action.
Normally hex occupies that bonus action and because it scales with number of attacks it scales with level as Elderich blast increases the number of beams moving from 1d6 all the way up to 4d6 at level 17. Hex doesn't scale with spell level apart from duration which is irrelevant to warlock as they get their spells back on a short rest anyway.
The new Spiritual Weapon also uses concentration and increases 1d8 in damage for each spell level meaning that by level 9 warlocks can get an effective 4d8+spell mod out of it rather than 4d6 without Cha mod by level 17. That's essentially 9 more dpr at level 9 vs with hex at level 17.
Is this a case that Spiritual weapons is a good? Probably not but it really demonstrates how bad Hex is most of the time.