Why MII interface needs two clock TXCLK and RXCLK where as RMII needs only one clock REFCLK
While related, they are different Media Independent Interface standards between Ethernet MAC and PHY.
MII : When transmitting, the PHY uses the local clock for the MII TX clock (and for the MAC) to send data, and when receiving, the PHY locks on to the received data stream and synthesizes the reception clock so the PHY sends the data with recovered MII RX clock to MAC. So the MAC reception is asynchronous to the system clock and it needs to handle it, as the clocks of different devices will not be identical due to allowed tolerances.
RMII : There is a single RMII clock between MAC and PHY, so the RMII interface and MAC are synchronous. Again the single RMII clock is used for transmission, but to allow clock tolerances, the PHY must recover the reception clock and buffer the data in a FIFO in order to send the data over the single clock to MAC.
RMII means Reduced MII and it has half the data path width so the clock is doubled to get same throughput, and some of the synchronizing signals are combined or just used multiplexed or otherwise slightly differently to reduce the interface pin count even further.