I know both Sisters of Battle and Space Marines employ a lot of brainwashing and mental reconditioning to their recruits to assure loyalty to the Emperor and the Imperium, but there was only one occasion I can remember of a sister ever falling to chaos, but a lot of instances where Space Marines did the same, from single astartes to entire legins no less. What can be the cause of it? both are pretty much faithful.
The Sisters of Battle are very pious and strict in their religious practices. Any deviation from the canon would be met with strict disciplinary measures, and falling to Chaos would be dealt with harshly. The contrast with the Space Marines is that the Astartes teach loyalty to your brothers, the Sororitas teach loyalty to the Emperor. The difference is that the latter leaves absolutely no tolerance for allowing little heresies for the good of the Imperium, and those who stand with you will be the first to deal with you if you were to ever fall to Chaos. And if individuals cannot fall, an entire convert of fanatically loyal Sisters of Battle will certainly not fall.
The biggest issue is the nature of the two groups. If the Astartes were religiouslyrics based then they would probably be like the sisters, however their basis is basically agnostic, they agree that the Emperor is amazing, but few of them think of him as "The God Emperor". And then their list of priorities starts with their Primarch or Chapter.
The Sisters in contrast are brought up from childhood in one of the most harsh forms of disciplined training imagineable. Where Marines grabs the strongest savage world teens possible, Sisters start extremely young with a daily regiment of prayer, combat drills, and scholarly education provided for by the state church. Where a Marine is told that their body makes them the greatest thing ever and almost unbeatable, a Sister is told that without the Emperor she is nothing and any deviations from her doctrine will make her little betterrain than the heretics her class burned on their last outing. Sisters thus do not have the skills, nor strength and technical skills of a Marine, but they have unquestionable devotion which is what the Ecclesiarchy holds in higher regard to actual combat prowess.