It's a big scandal because they cheated and were the champions. The World Series went to game 7, so they likely don't win without cheating. Why would they continue to cheat if it didn't work? Anyone who has played baseball knows that if you know what pitch is coming you have a much greater chance of hitting it.
The Dodgers are pissed because the cheating directly cost them a championship. Would you not also be pissed?
No one truly believes that the Astros were the only team doing this. We just don't and won't ever know how widespread it was. Likely only a few teams at most, not the entire league as Astro fans would have you believe. The Astros seem to have been the dumbest and most obvious about it, though.
The 2019 Astros were better than the 2017 Astros. Both teams were good enough on talent alone to win a championship though. The two biggest instigators of the cheating, Beltran and Cora, were gone by 2019. That probably played a big role in their cheating being stopped, if it actually did.