The problem with raisins arises during the drying process. Drying a fruit increases the sugar content dramatically and decreases the fiber.
As you probably know, raisins are dried grapes. A serving of grapes (4 ounces) has a glycemic index of 43 and a glycemic load of 7, both of which are on the lower end (Reference 1). However, a serving of raisins (2 ounces) has a glycemic index of 64 and a glycemic load of 28, both on the high end (Reference 1).
The key to eating for weight loss is to maintain a stable blood sugar. This strategy helps to keep insulin at a reasonable level and significantly reduces hunger and subsequent energy intake.
Despite having some nutritional upside, eating raisins is very similar to eating candy when it comes to your blood sugar. If you are trying to lose weight, a much better choice is to go for fruit in its whole and natural form.