Buy in stack memory

After going above the max buy in at a table, when rejoining, why can I only buy in at where I left off?

Is this bad design? Shouldn't I be able to choose from min buy in to where I left off?

E.g. I got mugged when leaving, so I can no longer afford the buy in at where I left off

This is because of a common issue at cash games called "going south." Winning a large amount and then leaving only to come back and buy in for a lower amount and banking the remainder. By prohibiting this it protects the money on the table (exception to disconnect like in your case where you got mugged) and the opportunity for players to win your stack.

There is a period of time where this resets. I am not sure of the exact time frame or if it also includes when the table empties out or not, but it does eventually reset.