As I already mentioned upthread:
For the same reason, you can't do a huge mountain stage in the area (unless you cross the border). The only Italian pass within 30k of Tarvisio is this, just under 20k away:
Alternatively, you could do the climb below, descend 15k and have approximately 20k of flat to the finish line (less if you finish as close to Lussari as possible, more if you finish in Tarvisio itself).
Both of those options are less favourable to whoever is the strongest climber at that point than the original MTT would have been, but they don't shift the advantage to the better time-trialists among the GC riders like pretty much any alternative TT starting in Tarvisio would.
And yes, a mountain stage like that would impact the preceding Dolomites stages - by making them better, because nobody is spending the queen stage thinking about saving energy for freaking Sella Nevea of all climbs.