Reason is obvious. Previous versions have all sorts of already fixed and well known security exploits - the one you used to jailbreak your phone is a prominent one. Despite new OSes having all sorts of bugs, it is still not recommended to use an old OS version due to this. You are putting your phone and your personal data at risk.
Think about it, in this age where your phone equals to most of your life, and even an ordinary user like you can hack (jailbreak) your iOS version, do you think a more skilled hacker can do the same? I bet there is a high chance that they can, probably without the need of physical access to your phone.
I remember some previous iOS versions can be hacked by simply receiving a text message. Not to mention earlier exploits like jailbreakme.com etc.