Because Trump is the first realistic presidential candidate in the history of USA whose any single view aligns with any single libertarian view (except JFK). So you either support a step closer to libertarianism, or leave your views in words only as always, thus effectively supporting status quo, which is extremely far from libertarian dream and being brought further and further away every day ever since end of slavery.
Not to mention that he has a proven track record of fulfilling every promise he makes.
To break it down to the basics, here is the choice that libertarians have:
1. Take a step towards your destination even though that step does not bring you to the final destination yet.
2. Do nothing and stay exactly where you are, unless others do something and push you to wherever they desire.
3. Attempt an impossible move straight to your final destination in one step at once by opposing all but your perfect candidate or other perfect solution that has no chance (and proven in history) of any significant support, and therefore does nothing except leaves you at point 2.

So point 3 is effectively point 2, but less obvious and could fool some well wishing unsuspecting libertarians. Unless you are quite happy with status quo and are afraid to try any steps at all not to ruine something, you should never choose point 2, much less point 3.

From my point of view, Trump's pro-libertarian stance is so obvious after his latest speech, especially keeping in mind that he also has not to scare away his republican and democrat supporters, that nobody can sincerely not see it. Thus I suspect only enemy agents can pretend not to see it.