Players still tried, but it was just not an efficient attacker because of its attack cost
Rukarumel, Biologist is not a sliver herself, which may seem strange for the head of a Sliver deck, but she does make slivers and other creatures into other creature types
When it resolves, you get to pick an opponent and dig through their library until you find a creature you want and you put that card directly into play on the battlefield under your control
I'd compare it to something like Gravecrawler + Phyrexian Altar
They can attack and tap as soon as they enter the battlefield, as if they had been out for a full turn beforehand
The other reason this card is great is that it can be copied easily, being non-legendary, which means you can achieve aerial superiority provided you have enough pilots around ready to fly