why some grapes have seeds and others do not?

Answer 1

All "natural" sorts of grapes have seeds. But in nature, not all hybrids can breed. For example, if you have a dog whose mother was a collie and whose father was a German shepherd, it will still be able to have puppies. But if you have a mule, which is the child of a horse and a donkey, it is sterile.
When you breed two sorts of grapes, they may be able to produce new plants, or not. Hybrids which can not do not produce any seeds. People have noticed that and bred this quality in hybrids so they are easier to eat. The same is done with other plants, like oranges.

Answer 2

Seedless fruit are geneticall modified in order to make them easier to eat.