If we are using 64 QAM adaptive modulation schemes, the higher the Modulation scheme the higher will be Bit error rate.
While higher order modulation rates are able to offer much faster data rates and higher levels of spectral efficiency for the radio communications system, this comes at a price. The higher order modulation schemes are considerably less resilient to noise and interference.
As a result of this, many radio communications systems now use dynamic adaptive modulation techniques. They sense the channel conditions and adapt the modulation scheme to obtain the highest data rate for the given conditions. As signal to noise ratios decrease errors will increase along with re-sends of the data, thereby slowing throughput.
By reverting to a lower order modulation scheme the link can be made more reliable with fewer data errors and re-sends.
I think this is the reason for limiting this up to 64QAM not only in LTE but in all radio communications we have to consider these things while adopting for Modulation techniques.