I think because it is a tangible thing one can do. Regardless of opinions on constitutionality of firearms ownership, nobody wants to see attacks and shoots. But what can be done? Changing the laws is not likely because of both disagreement and gridlock, though political activity is one avenue one can take with hope for longer term impact. Policing or mental health policy and programs in the in the local communities also might have some impact, but again most individuals won't be able to make an impact (not that they couldn't but won't in practice without a high degree of engagement).

But hosting a class? That is something everyone organization has full autonomy to successfully accomplish. It is within the organizational and budgetary resources of small companies, big ones, churches, schools, local community centers, etc.

Think about the duck and cover training for surviving a nuclear blast. You might not be able to do much as the average individual to avoid nuclear confrontation, but you can mitigate it on the margin with some tangible steps.