1 year ago, baby Cory was born

Last year in a newly set up 70lt I introduced a 4 C. juli (?) and after some time I started noticing a very little catfish hiding under a log, he grew up (see pic).
He was super cute!

Unfortunately the cory family Lasted not even a year, while other fish like Betta, minnows (ok, they’re super hardy) were fine and stil are.

How come they passed from reproducing to dying without any change in the environment?

Cories do best in a group of 6+ on a substrate of soft sand. Substrates that are sharp, or large enough to trap particles, harbour bacteria that infect a cories mouth and barbles.
Incorrect parameters (temperature, GH, pH) or deteriorating conditions (ammonia, nitrite, nitrate) will also weaken them.
Your Cory was a c.trilineatus.