I read this question as why OFDMA has higher PAPR, if we are clear then other is easy to understand -
In case of OFDMA, a multicarrier signal (OFDMA has many subcarriers which are orthogonal to each other) is the sum of many narrowband signals. At some time instances, this sum is large and at other times is small, which means that the peak value of the signal is substantially larger than the average value.
Now if we see SC-FDMA which is single carrier so this situation does not arise.