Robert Horning says:

It turns out that the Earth is fairly uniform in terms of its mass distribution (with a few exceptions…. there are mascons on the Earth as well, but they aren’t nearly so pronounced as the lunar ones… Antarctica being one of them on the Earth… that impact the Southern Ocean as well). One of the side effects of this is that there are only 4 stable orbital “inclinations” or paths that can be used to orbit the Moon for a long period of time. (See http://science.nasa.gov/headlines/y2006/06nov_loworbit.htm for some additional details) These orbits work at 27º, 50º, 76º, and 86º, with the last one that is perhaps the best one for lunar mapping efforts like the LRO.

NASA happen to get very lucky with Apollo 15 when a “sub-satellite” or a satellite was launched from the Apollo 15 command capsule while David Scott and James Irwin were having fun on the surface of the Moon. It happen to be released in one of these more stable orbits (at about 28º inclination from the lunar equator), while a similar satellite launched on Apollo 16 crashed almost right away. “Stable” is relative as well, but it is significant that this information is something that was discovered by the Apollo program and wasn’t even considered as something worth bothering about by engineers who had only previous experience orbiting stuff around the Earth.

BTW, after digging around, it appears as though the LRO is going to have an inclination of more or less 90º (with some adjustment of about a degree or two during its expected lifetime in orbit). I think that the engineers are sacrificing some time in orbit to get more accurate maps… and trying to map the polar regions of the Moon. It is hoped that water ice is going to be found in the lunar polar regions in what is called an “ocean of eternal darkness” (see http://en.wikipedia.org/wiki/Peak_of_Eternal_Light for details… cool names if I ever hears of some).