User blog comment:B214/Volume 22 spoilers/@comment-4547267-20160802061536/@comment-3435913-20160802234856

Asia is a follower rather than a leader, so she naturally leans on people for support. Additionally, as someone devout in her belief to follow her leader, she is willing one to do a lot of things if they told her to.

Xenovia was raised to be a subordinate and performs better serving under someone than when she acts on her own. She is also a strong believer in following orders so it's not a stretch.

Ravel was raised in a noble family so she's always been told how to act, what to do, and what is expected of her, so again it's not a huge stretch. In addition, since the peerage system is an integral part of devil society, and was already part of a peerage before the series, she's already very familiar with how the system works.

If you're raised as part of any system (whether it be an organised religion, a well standing family, or actually just part of society) you're naturally in tuned with the ability to follow orders and fall in line (following the law counts, since people are aware of the consequences of breaking them).

Issei, who is honest to a fault, immensely powerful and renowned, and is willing to put his life on the line for others offers the perfect situation desired in a leader since the trade off is loyalty and subservience which is fair considering the benefits proposed by the opposing party.

But the most important factor is that they love Issei and want to stay close by his side.