There should've been a reason provided. If there wasn't then you probably didn't provide enough evidence that this would be a good game. This often happens if you don't record a long enough video with your submission.
Roblox games get redirected to the games forum because they handle them there.
So all rejected roblox games get sent there. You ignored the link provided to you in the reject message.
So what was the reason for making the thread then? They redirected you to where you submit Roblox games and where they get handled at, should've been explanatory enough why it was rejected from the normal game requests.
Judging by the responses here, you submitted your ROBLOX request through the game requests feature for the whole website, used primarily for games that are not part of a series. ROBLOX is a fairly large series with 127 leaderboards and has a series moderator to handle requests for the game at a lower level. This is a logistical move to help alleviate requests for games that are similar in nature.
You were likely redirected to the ROBLOX series forum, which has a separate games submissions form made specifically for ROBLOX games located in this thread: https://www.speedrun.com/roblox/thread/vii9z
Yeah I'm sure you do know. You were directly linked three times (four if you count the above posters link) to go to the thread in the forum for Roblox, please stop wasting our time. Thank you.