It is strategy, they want to use money to finance other future projects. According to one interview I have read, money from Minecraft are used to pay bills and development of other games until they luck out again and have another hit.
Developing games is risky and there are no guarantees of success. So, the successful projects have to pay for unsuccessful ones.
Just to add to your last sentence, successful projects have to pay for projects not yet ready to release as well as unsuccessful ones (whether due to them flopping or lack of critical mass.)
Developing games is risky and there are no guarantees of success. So, the successful projects have to pay for unsuccessful ones.