OFDM takes advantage of frequency diversity to achieve higher data rates
In order to cope up with the multipass, delay-spread and inter-symbol interference associated with the Single Carrier(SC-FDMA), OFDMA symbols are made of long duration symbols and multiple sub-carriers with the data symbols made to modulate separately on diff sub-carriers, therefore it is very robust in terms of delay spread and inter-symb-interference. It is because of this virtue, it is used in the downlink, which carries demand for higher datarates.
Whereas the same thing is compromised on the uplink by using SC-FDMA because of the significant disadvantage of much higher PAPR(Peak to Avg power ratio) associated with OFDMA, causing a high battery consumption. However the Basestation is able to cope up with this as it is much more power-abundant as compared to the UE.