Because the tulip mania was the first ever bubble in the history. And it became the benchmark for everything the economist think of a rapid increase in price followed by a crash downward trend. The uptrend is driven by speculation and irrational behavior of the market. Just like what happen to dotcom bubble back in the last 90's to early 00's.

But you have to think that there are some serious flaws in the argument. Bitcoin has limited supply as compare to the Tulip mania back then, when everyone can literally have harvest their own tulip in their backyard. That's how the bubble burst because the supply is unlimited as compare to bitcoin which has only 21 millions coins.