West Africa and the affected nations are /hardly/ an entire continent, let's not get too dramatic J.
The problem is, the affected nations have rather porous borders, and the entire region suffers from civil administration that is somewhat rough around the edges. Quarantining just Guinea, Liberia, or even "West Africa" generally would simply encourage travel via more convoluted routes through other countries and regions, and this would make potentially infected travellers more difficult to track. Once such travellers started to reach, say, the USA via, Kenya, or South Africa, the present hysteria would probably demand quarantine for the entire continent.