It’s because almost no goalies under 6’1” are able to hold down an NHL starter role anymore.
For example, this year every team has played 36-40 games, so let’s say 18+ games played is a starter. 30 goalies have played 18+ games, while only one, Saros, is under 6’1”.
Last year, 30 goalies who played half+ the games, just two under 6’1” (Saros and Nedjelkovic). The year before, 27 goalies played half+ the games, again just two under 6’1” (Saros and Khudobin).
So the reason ppl are doubtful that sub 6’1” goalies will be able to translate their game to the NHL is because they nearly always do fail to translate. You have to be an extreme outlier to make it as an NHL starter if you’re under 6’1”, there’s only 1-2 such goalies per year.