Because most of the routes on which they might be considered suitable are operated by TfL-owned LTs. Tri-axle buses are hardly novel,, any more than articulated buses, but voluntary take-up by the operators has been almost non-existent. As for air con failing, I have little knowledge on the likelihood of that: what I do know from so many official and unofficial reports is that battery (as opposed to diesel) usage on LTs is worryingly small, no doubt contributing to the disastrous emission figures for Oxford Street which were a precursor to depriving this famous street of its bus services in the near future. In short, bus manufacturers' promises must sometimes be taken with a pinch of salt.