I wishlisted that game, it's never on sale. Why ?

yes because it is worth the price :)
So much so that the price has been increased !
https://isthereanydeal.com/game/towerfallascension/history/?shop%5B%5D=gog&generate=Select+Stores

For me this is ok ...

But seeing that on Steam there had been real sales is strange - to say it mildly.

GOG should care - but I don't think any experienced GOG customer will expect this.
It is GOG which should look for playable games (we do have a lot games not working
under GNU/Linux still sold as available for Linux ... not correct!), latest versions
(there a really a lot forum posts about missing version available on steam for months ...
with luck one can get those patches after years), succesors and derivateives
(TowerFall for 8 Players, to give an example) and get in contact to developers
to make is accessible for their customers - and put those on sale ...

So in short - TowerFall is on sale frequently - just not for GOG customers.
Thank you!
But it may be worth the price to not buy from Valve ...