What makes you think these marriages are successful? Low divorce rates are not good evidence in places where divorce is often impractical.

Three main points in favor of arranged marriages that I'm aware of:

1. The marriages are generally arranged by older women, who are likely better at finding a long-term match than young people. (Consider this the equivalent of dating people based on okCupid match rating, say, instead of hotornot rating.)
2. The expectations people have from marriage are much more open and agreed upon; like Prismattic points out, they may have a marriage that a Westerner would want to get a divorce in, but be satisfied. It seems to me that this is because of increased realism in expectations (i.e. the Westerner thinks the divorce will be more helpful than it actually will, or is overrating divorce compared to other options), but this is hard to be quantitative about.
3. To elaborate on the expectations, in arranged marriages it is clear that a healthy relationship is something you have to build and actively maintain, whereas in love marriages sometimes people have the impression that the healthy relationship appears and sustains itself by magic- and so when they put no work into maintaining it, and it falls apart, they claim that the magic is gone rather than that they never changed the oil.

I also think most modern arranged marriages involve some choice on the part of the participants- "meet these four people, tell us if you can't stand any of them" instead of "you will marry this one person."

I remember seeing studies that attempted to measure happiness.