Why is SATA simplex even though it has separate TX+/- and RX+/- pairs? Why can't it be full-duplex like SAS (which has the same interface with multiple ports and is full-duplex). If I am wrong with this concept please correct me.
For backward compatibility with PATA/ATA/ISA, SATA uses the PATA/ATA/ISA command set. PATA/ATA/ISA is simplex because it is a connector to the ATA/ISA/PC/XT bus which is simplex. The ATA/ISA/PC/XT bus was simplex because it was a connector to the 8086/8088/80286 processor data bus, which was simplex.
SATA is consumer grade electronics it must come cheap with huge capacity. SAS is enterprise grade electronics, they care about speed, reliability and much less about money. Check the price and capacity difference for SATA and SAS drives. So my guess is price to build it.