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Not giving OP a (You) since this is a shit thread and very possibly bait, but for anyone else lurking that wants an answer to OP's question, the most accurate answer is simply that Rollout was a new move at the time of GS. It was completely new to the series, and no Pokémon up to Whitney's battle could be encountered with the move (the only exception being Jigglypuff, which the player would have to catch and train to Lv19 to gain the move. This was somewhat feasible as Whitney's mons are 18 and 20, but Jigglypuff was rare, only available in a small side area).
Most players simply didn't know how the move works or that it even existed - you have to remember that even after the game's release, information traveled more slowly at the time, and not everyone had Internet access. This is the same reason myths like "Mew under the truck" were able to gain so much attention. Combine this all with the fact that Miltank has decent ATK at the time it's encountered and Rock is such a strong type offensively, it makes sense that Miltank's Rollout provided a somewhat significant challenge for players in their first playthroughs.
