Behemoth + Sidekicks okay but Manglers not...?

I’m genuinely curious as to why Sidekicks are okay on Behemoth yet Manglers aren’t. Why is that? Sidekicks on Behemoth creates the most slow paced game ever and are arguably more close ranged than Manglers. I don’t understand the choice for maps and weapons in Tactical Slayer. It makes no sense to me at all.