Some apps are growing wise to holdout users, and force you to update to continue using services. That's ultimately been the death knell in every jailbreak effort I try, some essential app will eventually force me off the jailbroken iOS version.
That’s not necessarily out of malice, though. Quite possibly the older version is using some server endpoint or format that the developer has deprecated and they want to reduce their maintenance burden.
It is not malice, it's laziness and stupidity. Most apps are so trivial it is absolutely not necessary to break compatibility when you release a new version.
Depending on the app, it's just sound security. On a jailbroken device, you cannot assume any of the OS safeguards are still in place. On a stock iOS device, there's pretty much no relevant malware that could be on that device. On a jailbroken device, all bets are off.