why more and more billet compressor wheels on the turbo market than cast?

There are many benefits to this type of wheel manufacture because billet compressor wheels are made from a solid piece of alloy(billet) A CNC milling machine . 

1.     Strength     solid piece of material rather than via molten material .billet generally has less flaws. This extra strength allows the wheel blades and hub to be thinner, increasing blade area resulting in increasing blade area resulting in increased air flow from a wheel with matching overall dimensions.

2.     Weight       as the billet compressor wheel can have thinner blades and hub, this means that generally the billet compressor wheel will also be lighter than its cast counterpart.

3.     Modification   As each wheel is individually machined via a CNC milling machine and CAD program, rather than the multi piece casting tool, it allows the change of the design to modify the wheel EG. Adding extra blades ,extended exducer tips etc. A one off design now does not need to involve making an expensive dedicated tools to cast .