Well, at the risk of venturing into the political, the Washington Post is a rather biased source, and like any media outlet, requires a good bit of critical thinking when you read it. They are very open about their bias in their reporting.

There were large numbers of military already engaged in other disasters and operations:
Afghanistan is ramping up
Iraq/Syria is ramping up
North Korea is ramping up
Forces were already in Houston for disaster relief
Forces were already in Florida and the Caribbean for disaster relief
Forces were on standby for Puerto Rico before the hurricane hit

Other responsibilities and missions do not end, and disaster assistance is not a real mission of the US armed forces, it is a capability that is used, but they prepare and resource for missions.

The US military does not have unlimited resources. Comparing with an earthquake is like comparing apples and oranges. The differences in the types of damage and how widespread are great. For an island where the airfields, ports and transportation infrastructure is so heavily, and universally, damaged; and the neighboring territories are similarly damaged, it is a good deal more difficult. The time period of the Haiti earthquake was when the missions in Afghanistan and Iraq were winding down, so more forces were available. There were also not back to back disasters, it was an isolated event. The population of Haiti is much larger than that of Puerto Rico, as is the size.