The release of Conor Washington was certainly the biggest surprise in the end of season retained list: https://londonnewsonline.co.uk/conor-washington-the-most-notable-name-in-charlton-athletics-released-list/
Perhaps the lack of goals was an issue, but he contributed in other ways and had a good partnership with Stockley. An alternative explanation is that he was made an offer but turned it down as not good enough.
Among the other decisions, the retention of injury prone Ryan Inniss has been controversial, Forster-Caskey is probably a better bet on the injury front. Josh Davision has scored nine goals at League Two Swindon, but is probably being retained to sell on, although he may stay until January as cover. More here: https://londonnewsonline.co.uk/charlton-athletic-trigger-contract-extensions-for-four-players-including-forster-caskey-and-inniss/
Blackpool could still be interested in Charlie Kirk, but at a lower fee than £500,000: https://londonnewsonline.co.uk/blackpool-still-interested-in-summer-deal-for-charlton-athletic-winger/
Lots of potentially interesting players have been released elsewhere.