I don't know why there is no trial version of LabVIEW for Linux but if you want to try one on trial, then you can contact the local sales engineer of your area and maybe he/she can help you out. To get information on the engineer, just contact your local NI office.
My understanding was that the activation mechanism used in Windows wasn't ported to Mac or Linux (presumably because it was considered too costly). This is what NI uses to allow temporary access to the full version.
