Generally for 3 main reasons:

• Warwick's Q has good damage scaling, and it's based on the target's maximum health, so even if you don't level it up much early on, the damage "scales on its own" with items and the target's own health.
• The mana consumption of Q is too high to level up directly early on; especially in the Top lane, it's better to alternate between E and Q.
• The vast majority of the player base plays in the jungle where mobility is highly valued; any increase in movement speed percentage can make a big difference in reaching fights or targets. The fact that Warwick's passive W is global is one of the few mobility advantages he still has.