OFDMA requires complex DSP processing which was not evolved till late 90's.
To Explain it futher ,
Multicarrier communication systems were first introduced in the 1960s with the
first OFDM patent being filed at Bell Labs in 1966. Initially only analogue design was
proposed, using banks of sinusoidal signal generators and demodulators to process the signal
for the multiple subchannels. In 1971, the use of the Discrete Fourier Transform (DFT)
was proposed , which made OFDM implementation cost-effective. Further complexity
reductions were realized in 1980 by the application of the Winograd Fourier Transform
(WFT) or the Fast Fourier Transform (FFT).
The first cellular mobile radio system based on OFDM was proposed in 1985. By then,
the processing power of modern Digital Signal Processors (DSPs) has increased remarkably,
paving the way for OFDM to be used in the LTE downlink.