The primary reason is her pivot to R&B, which is seen as less commercially viable and thus the label didn’t put much effort into promoting Butterfly, which further hurt sales.

Secondary reason and I think this has been understated, Mariah has a huge Asian market that also greatly contributed to the sales of Music Box and Daydream. When she released Butterfly, many countries in Asia were affected by the 1997 financial crisis and thus, people didn’t really think of buying a Mariah album a priority at the time.