The purpose of both these tools is same but how it delivers is different. eCATT is native ot SAP and you do not need a separate installation also, you may need the knowledge of the ABAP to code. It also has most any of the features of the QTP but is not easy to work as QTP.
Also remember teh fact that QTP is from a third party and you have to buy the product to use it. QTP comes with an SAP plug in for it to be used on the SAP application.
Most SAP implementations use QTP. I do not exactly know why, but I can hazard a guess. If a guy happens to have the knowledge of ABAP, he would rather well get into the SAP technical side itself. I am not sure if some folks out there would be specifically be interested in getting into testing. I will leave it to any one's guess here.
I hope this answers your question.