Hi,
In SPI, the data is sent out and received synchronised with a clock and even to receive a byte, you have to send out a dummy byte where as in a UART the data TX/RX is done asynchronously using start stop bits to package the data bytes in transmission. So, for a byte TX/RX, it requires a minimum of 10 bits for UART.
SPI is based on a master slave concept where it is always the responsibility of the master to issue clocks and send or receive a byte. There need not have any such relationship between two devices communicating over a UART.
Since these two systems are inherently different, they can not be connected directly, you need a converter in between.