The main reason it wasn't escalated sooner is that through January China was still withholding and delaying the release of data. China only confirmed that cases of human to human transmission had occurred on January 10th, and froze reporting of new cases during a party congress that ended on January 17th. So the WHO escalated this appropriately within 2 weeks of the first indications from China that this was a serious issue, despite confusing and inaccurate reporting of data.
Also about 2% mortality was an early estimate, is 20x that of most strains of flu and is comparable to that of the Spanish Flu pandemic. So those estimates were as good as they could have been at the time, and are still not far off what we're seeing happen.
[0]https://www.who.int/news-room/detail/30-01-2020-statement-on...
[1]https://www.thelancet.com/journals/lancet/article/PIIS0140-6...