They buy from them for the same reasons others buy from you. But there may be reasons that are not known to you.
You may look at a best-selling book and wonder why it sold so well or you may look at a great book and wonder why no one has heard of it. Look at your competitors, whether they are authors, consultants, web sites, DVDs, etc. and analyze what they offer that is so appealing. See if you can copy their offering or exceed it.
The things with which to compare yourself can vary, but certainly you should see how you measure up on: