Installation on 64bit Linux systems requires 32bit libraries, which can usually be installed with one or two commands:http://www.ribbonsoft.com/en/qcad-installation#linux
QCAD depends on various libraries underneath to provide core functionality. Not all of these can be compiled or run stable as 64bit versions. Contrary to popular belief, 64bit does not bring any significant advantage in terms of performance which makes this potential rather large effort on my side merely a nuance for most end users.
Having said that, as soon as 64bit becomes simply a question of recompiling sources, there will be nothing in the way of a 64bit QCAD release.