Intention of an eNodeB is to send as much as data possible in downlink. By using OFDMA technology, eNodeB can achieve this. OFDMA consumes more power compare to SC-FDMA. Since UE has limited power (battery) that's why it uses SC-FDMA technique to send data in uplink direction.
Conclusion is when eNodeB has more power and it can use OFDMA then why it would use SC-FDMA which provides less data rate.
If you check the LTE downlink and uplink data rate then you can easily make out due to SC-FDMA, uplink data rate is less.