One factor I've heard cited is that if you can prove something in calculus using nonstandard analysis, you can always prove the same thing with standard analysis.

If we switched to teaching calculus based on nonstandard analysis we'd have to also teach the standard approach because of all the existing material that uses it and all the people who already know the standard approach but not the nonstandard approach.

Not many people are willing to commit to a few generations of teaching dual approaches and using both in their work until the people who only know standard are all dead or retired and all the old material has either been translated or is obsolete, when the advantage of the nonstandard approach is just that it might be conceptually easier or more intuitive.