Because being able to tutor anything for 3 mana lets you play your cards. Slivers are toolboxy, especially in EDH.

i.e. Opponent is playing control? Let's search up [[crystalline sliver]] and give my dudes shroud. I'm mana starved? lets grab [[manaweft sliver]] or [[gemhide sliver]] to give me massive mana. Do i need to destroy a pesky artifact or enchantment? grab [[harmonic sliver]]. destroy a land? [[necrotic sliver]].

sliver queen is only ever there to summon 1/1s. [[sliver overlord]] can be anything. Combine sliver overlord with [[Amoeboid Changeling]] and overlord now lets you take control of target creature for 3 mana once per turn. There are other spells that can enable this but amoeboid changeling is technically a sliver so he can be tutored by the overlord as well.

[[sliver queen]] is a finisher. overlord is an enabler. You use the queen to kill people. [[Gemhide sliver]] + Sliver queen + [[basal sliver]] + [[heart sliver]] is infinite haste slivers. combine with [[psionic sliver]] or [[screeching sliver]] and you don't even have to swing to win the game. Good luck trying to get this off without the tutor ability of the overlord.