Why did so many people have trouble with whitney's milktank back in pokemon silver?

I just simply used my flaffy's thunderwave to hold down its speed down and stop its rollout attack and used thundershock though flaffy and bellsprout went down I was able to use Flash to lower milktank's accuracy and make rollout less of a threat then finished the battle with noctowl who was level 22 Female using peck and tackle. But I don't really see why it was so hard to beat even though I only won because I used strategy XD.