Because they try to get round the perceived drawback of traditional funds.
Trackers do something very different.
They attempt to track the performance of the index itself, rather than set out to beat it. This is because they believe that in the end, no traditional manager manages to do better than the index for very long, and if that is so, it makes more sense to literally track the index. Either by buying shares in all the companies that make up the index, or by using complex financial instruments to track what the index actually does. Since tracker firms do not need to have expensive teams of experts who need to follow what individual companies are doing, they are cheap to run. They are cheaper still because they do not have the costs involved in buying and selling different shares on a regular basis.