It actually has nothing to do with the security of iOS. It has to do with two issues.
1) Jobs hated widgets because they were busy work and clutter.
2) They would have to keep iOS awake longer which uses more battery.
#2 is the interesting one. To be useful on the lock screen you would have to periodically update it. Otherwise the data would be out of date until you hit the standby or home button and then give the widget time to update. So a quick glance won't cut it because you could be reading old data when you do look. BTW that is how the weather widget in the notification area currently works.