Because many of the researchers became obsessed with the new information that they were observing from Chimpanzees. In fact its often stated that Bonobos and Chimps are basically our Dr Jekyll and Mr Hyde.
In fact, despite been genetically further away from us, Orangutans are actually seen to be our closest relative in behaviour and by far in intelligence. Bornean Orangutans may actually live an existence similar to early hominids in that they travel far on the ground, however Sumatran Orangutans show more of our social instincts including tool use and group learning (they actually teach each other, opposed to the Chimp method of watching from a distance when new behaviours are exhibited).
The other reason for the radical changes is that Westerners had less access to the middle eastern countries to study the Asian primates (that like Orangutans can show us traits that were common before the divergence of the groups). Also the interest in Chimps is decreasing as they're increasingly being seen as predominantly over aggressive, which is actually an extremely uncommon trait amongst our close relatives (Orangutans and Gorillas are most dangerous to humans, not through aggression but through fear, where as Chimps are dangerous through direct aggression).
Orangutans are known for calculated reciprocity, a behaviour only exhibited in Humans and never exhibited in Chimps despite them being better studied and closer related. It shows an ability in Orangutans to track individual relationships in a group over long periods of time and to gauge how they should respond to individuals. Basically if you bought me a $30 gift, I'm not going to go out and spend $300 on you, and Orangutans do the same.
This leaves many questions like is it a second evolution of a similar behaviour, or did our common ancestor have this trait and the Pan and Gorilla genus both lose this ability. Either way it makes Orangutans a very important animal to study, because potentially they are our closest relative by means that they might be evolving along a similar path as humans.
