The sea is enormous but you can only use all of that room to avoid collisions if you aren't trying to get anywhere in particular. Just like you are more likely to see a lot of airplanes over an airport than in the rest of the enormous sky, there are some choke points in the oceans where traffic is concentrated. Probably the worst of these is the Strait of Malacca. This collision happened very close to the Eastern entrance to the strait.
The following is from Wikipedia.
"The strait is the main shipping channel between the
Indian Ocean and the
Pacific Ocean, linking major Asian economies such as
India,
China,
Japan,
Taiwan, and
South Korea. Over 94,000 vessels
[3] pass through the strait each year (2008) making it the busiest strait in world,
[4] carrying about 25% of the world's traded goods, including oil, Chinese manufactured products, and
Indonesian coffee.
[5] About a quarter of all oil carried by sea passes through the Strait, mainly from Persian Gulf suppliers to Asian markets. In 2007, an estimated 13.7 million
barrels per day were transported through the strait, increasing to an estimated 15.2 million
barrels per day in 2011.
[6] In addition, it is also one of the world's most congested shipping
choke points because it narrows to only 2.8 km (1.5
nautical miles) wide at the Phillips Channel (close to the south of
Singapore).
[7]"