Why WHO has not eliminated chicken pox like smallpox?
Eliminating a virus from the world is an immensely costly undertaking. As with most things in real life, cost vs. benefit (and feasibility) need to be taken into account. Unfortunately, there is a limited amount of money available to fight disease.

The overall mortality rate of smallpox is about 30%. That's a very high mortality rate, one that shaped history significantly.

By contrast, of a yearly 4 million people who contracted chicken pox (in the US, before the vaccine), only 100 to 150 died each year.

The death rate of smallpox: 30%. The death rate of chicken pox: .00375% 

Maybe someday chicken pox will go the way of small pox. But our limited monies need to be devoted to eliminating viruses that cause greater harm, e.g. polio and others.

Also, not all viruses are equally "eliminatable", e.g. H1N1, HIV, etc.

Smallpox Vaccine: The Good, the Bad, and the Ugly
Monitoring the Impact of Varicella Vaccination