I have a feeling that most assume making stuff a higher light level is simple like a flip of a switch. To revamp the old raids would take a lot of time going through a lot of code just to ensure that drops and enemies and play through all work seamlessly together. Also the debugging process alone is a lot. If you noticed with the new April update they didn't really increase the light much of the original POE all they did was add two more stages and about 16 challenges. For all you know they could be planning to bring back the old raids in the Fall update but creating game takes time.