Why did the WHO "raise the alarm" about omicron?

As stated, this strain is likely to spread around the world, and a larger increase in the number of infected could have severe consequences in some areas.
Although it is not yet known to what extent omicron is contagious for those who have been vaccinated and recovered from COVID-19, the WHO recommends accelerated vaccination of risk groups.

Europe in Fear of the New Covid “Botswana Mutation”: What do we know about it

As of today, governments across Europe are rushing to step up their defenses against a possible new coronavirus outbreak after South Africa announced the discovery of a new strain of Covid-19 that scientists fear could thwart the pandemic effort.

Assoc. Prof. Glomb: Variant of COVID-19 of Bulgarian Origin May Appear Soon

If we continue to have low immunization coverage, I would not be surprised if a variant of Bulgarian origin appears. Upon entering a host, the virus seems to borrow life and begin to dictate conditions. The more passages pass from person to person, the more we allow the virus to improve. That's why these variants appeared.