It's true that there are 8 bits in a byte as stored in memory. But SATA uses 8b/10b encoding
so it actually takes 10 bits to transmit a byte over a SATA connection.
Therefore, a SATA connection running at 1.5Gbits/sec can transmit 150MBytes/sec. It's actually even a little less than that if you take protocol overhead such as headers and checksums into account.