Why do some games take a little more than 20 minutes while others go on for an hour.

there are many reason for this...

if you fed an assassin well the chances are he will snowball end game fast. Some games has many scaling champ, they take up to 30 mins to be really effective (if your team manage to stall the game)

sometimes in solo q, team has a really good communication which can lead to acing each other. (these makes it long i guess)

sorry for my bad english i really did tried

Because waveclear champions have the ability to stall out games.

Game length varies based on the coordination of the team, their champion's abilities to win early vs. late, the responses and coordination of the enemy team, whether early game or late game objectives are focused, and a million other factors. The "good explanation" is, basically, that game length is determined by how each team plays in terms of performance that game and playstyle in general. Because the game has so many moving parts even identical champions with identical builds in two different games could have a 30 minute difference in game length.