People are giving some answers but i think realistically its a mix of:
-It was simply not ready in time for champs this year, no point making promises for next year yet.
-they want the majority of people watching live on their streams with sponsors, FOMO is mitigated if you have a cooler viewing experience id say
-they dont want replays flooding their servers. From what ive gathered, valorant is never fully client side, a replay might as well take as much processing power as a regular match (they said the range is as costly as a match so go figure). 100,000 people watching the replay is equivalent to 1,000,000 player increase on the servers, and its for 1-2 hours each