Is CDMA more secure than LTE or GSM?
LTE (and GSM I believe) uses OFDM (orthogonal frequency division multiplexing) and simulataneously transmits several different "carriers" each containing a fraction of the full data bits. Upon reception, these demodulated fractions are then combined to restore the original (and full) data.

This means that each "carrier" is detectable in its own right. Compare this with CDMA where one of the significant military benefits is that any one particular "carrier" has a power that is close to or below the local ambient noise level in that part of the spectrum.


  I was curious to know why in defense applications CDMA is being
  preferred over LTE


This immediately means that unless you know most of the frequencies used in a CDMA transmission you will likely not be able to detect the presense of a transmission.