Here's why:
* Language evolution is gradual: Word usage changes over time, and it's not a sudden switch.
* Style guides vary: Different style guides have different preferences. Some might still suggest a hyphen, while others don't.
* Common usage: Over time, the use of "multitasking" without a hyphen became more common, ultimately leading to its acceptance as the preferred spelling.
However, you can say that around the late 1990s to early 2000s, the hyphenation of "multitasking" started to decline significantly. This aligns with the rise of personal computers and the increasing focus on the concept of multitasking in technology and everyday life.
Ultimately, the most important thing is to be consistent within your own writing. If you're unsure, check your preferred style guide or consult a dictionary.