Tom Welling Didn't Want to be on Supergirl

Maybe if Supergirl lasts enough seasons he will reconsider eventually.

I seem to recall when Smallville was starting up, people were asking Dean Cain if he would ever guest star on the show, and he wanted nothing to do with it. As you may know, Cain eventually did appear on Smallville as a villain of the week.
The only concern I ever had about about Welling appearing on Supergirl was that I didn't want him to play Clark, but that became a non-issue when they cast Tyler Hoechlin.
Around season 5 or 6, I wouldn't be too surprised if Tom Welling shows up on Supergirl in a one-off guest spot. Or not.

That's quite sad. I think they easily could have given him a role like Jeremiah or Eliza who don't really have much superhero or villain activities themselves.

Maybe if Supergirl lasts enough seasons he will reconsider eventually.

It's clear that Tom wants nothing to with anything Superman or the superhero world for the matter, despite the fact that he will be on Lucifer. It puts him in the category of actors not wanting to do anything with the superhero genre, which ironically that Smallville played its part in starting along with movies from Batman and Marvel.

He's trying to avoid typecasting

Rom I get your point just as Melissa is trying to avoid typecasting herself which is why she has done the other projects she has like Patriot's Day and the new Spike/Paramount Network mini-series Waco. This despite her enjoying being Supergirl.

To put it mildly, I don't blame him - but not for reasons you might think.

Tom played Clark Kent for 10 seasons and did his bit for God, (King/Queen), and country. There would be no need for him or any real motivation for him to put on the cape. The only time we saw him do this was in the final episode of Smallville and throughout the show's run, he steadfastly refused to do it despite fan and studio desires to have him do it.

For him, this aspect of his career is over and even though It would have been nice to see him do a cameo or play a minor role on Supergirl for nostalgia's sake, he more or less stated that his time in that particular superhero spotlight was over and done with.

As an actor, he doesn't wish to simply be remembered as the "guy who played Clark Kent." He's trying to avoid typecasting and it is a very challenging thing to do, especially with the weight and responsibility that a Superman (or any iconic superhero role) stint brings.