RVSM compliance et al - if you are shepherding an airliner or just intercepting one it is best if you don't break the congested & carefully choreographed airspace by introducing a non-compliant aircraft to the mix.
As for 1435 Flt they were not Tranche 1 jets in my time, even from the start of the transition from F3. It could have happened since of course but it was never the plan.