According to Ubisoft, the reason for the delay is to provide gamers with a “more refined experience”. The expansion was originally expected to be released towards the end of April, the 29th of April to be exact, but now it looks like the expansion has been pushed to the 13th of May, which is a couple of weeks later.
https://twitter.com/assassinscreed/status/1382363166982688769
To be fair, this delay is only for a couple of weeks so it’s not that much longer of a wait. This is versus other games that have been delayed for months, so it’s a minor inconvenience at most. Also, it’s not a bad thing to be delayed. Games that are rushed are pretty obvious and usually this results in less than optimal experiences.
A good example would be CD Projekt’s Cyberpunk 2077, which apparently despite the delays was still considered rushed, resulting in a hot mess for a game that was so highly anticipated.