Why do some plant based milks require more emulsifiers than others?
Some plant products carry a good amount of lecithin, which is itself a potent emulsifier, so there is less need to add an extra emulsifier. Soy beans and sunflowers are both commercially exploited sources of lecithin isolates, so one can assume they have a lot. 

Lecithin is also contained in eggs in a high concentration, and the reason eggs are often helpful in mayonnaise making - BTW, eggless mayonnaises can be homemade from soymilk, though not as easily as traditional mayonnaise.