Does Jaylen still not have an agent? This is something that might hurt him here. It made sense when he was a lock for a top 8 pick, no real negotiating going on with that contract, but now there will be a lot of negotiating when it comes to his next contracts as well as trade rumors. An agent can really help read the tea leaves and also find him a team willing to pay him and possibly help facilitate a trade there (not that I would root for that).
As for the max contract, I don't know, a lot of guys in his position (good player with potential to be an All-Star) don't get them.
Kemba Walker took $48m/4yrs (max would have been $90m/5yrs) after averaging 18/6 his 2nd and 3rd season.
Victor Oladipo took $84m/4yrs (max would have been $105m/4yrs) after averaging 18/4/4/1.6 and 16/5/4/1.6 his 2nd and 3rd season.
Aaron Gordon took $76m/4yrs (max would have been $147m/5yrs) after averaging 18/8/2 and 4 straight years of increasing output.
Zach Lavine took$78m/4yrs (max would have been $147m/5yrs) after averaging 19/3/3/1 and 17/4/3/1.
Clint Capela took $90m/5yrs (max would have been $147m/5yrs) after rumors PHX was offering a max or near max and averaging 14/11/2
Steph Curry, Al Horford, Joakim Noah, DeMar DeRozan, Jrue Holiday all guys who didn't sign max rookie extensions and it's not like these guys didn't show potential or were late bloomers. Horford was already an All-Star, Noah was already All-D, Holiday would become an All-Star the first year after signing an extension, DeRozan and Curry the 2nd year...
Sure you can also point to a lot of comparable or worse guys who got max or near max contracts, but Jaylen is definitely not a lock. Especially if he doesn't have an agent to help play hard ball.