The reason is very simple. The M.2 interface on your Z97 U3 PLUS mother board is according to the specifications, PCIe 2.0 x2. The Intel 600p, like most M.2 NVMe SSDs, must be used with a PCIe 3.0 x4 M.2 interface that provides 32Gb/s.
The detailed specifications of the M.2 slot on your board is: M.2 port supports M.2 PCIe module up to 10Gb/s speed.
Very few Z97 mother boards have M.2 slots with a PCIe 3.0 x4 interface. We must be careful to check the specifications of the M.2 slots on a mother board, to be sure they match the bandwidth requirements of the M.2 SSD we plan on using, sorry to say.