Why did whalers come to Hawaii?

Basically, whalers came to Hawaii because it was there. 
To be a bit more serious, they came there because it was a good place (the best place
between California and Asia) for them to restock their ships with things like water and
fresh food.

In the days of whaling, the whalers had to
spend months away from their home ports.  It took a long time to sail across the ocean
and so they would not want to be going back and forth any more than they had to.  But,
at the same time, they could only carry so much water and fresh food and so they would
need to replenish those supplies.

If you look at a map of
the Pacific, you'll see why Hawaii was a popular place.  The islands of Micronesia are
your only other choice in the Northern Hemisphere for places to stop and they tended to
be way too small to be able to support the number of whaling ships that were out
there.

So Hawaii was an important stop because of the size
of the islands, their location, and the fact that the technology of the time required
ships to stop somewhere to replenish supplies.