In many states it is possible to get an absentee ballot for any reason, which allows the same coersion potential but without the convenience of full vote by mail. Forcing voting in person also has a large effect on who actually votes (due to work or other issues getting in the way, which could also include coersion from a spouse who thought you might vote the wrong way or doesn't think you should be able to vote). Presumably people are checking if there are large swings in voting patterns when vote by mail is started that would indicate widespread coersion. Coersion can also be effective in practice even when the actual voting is done in secret.

I wouldn't recommend vote by mail for Mexico, where there is widespread vote buying even with secret ballots, but for the US it seems to me that vote by mail is likely to be more representative and increase the chance that voters will research the candidates as they are voting. No system is perfect so it is a question of what tradeoffs seem to make the most sense for particular situations.