Mongolia seems an ideal place for US North Korea dialogue. It has good relations not only with North and South Korea, but also the US, China, Japan, and Russia. It may similarly offer a more temperate political climate for the six party talks held in Beijing, if China does not object.
The US has looked to China to carry its water on North Korea. And in small measure, it has, but on the bigger issues Washington and China go separate ways.
So, Ulan Bator looms larger as as near a 'neutral' venue for Washington and Pyongyang talks.