Because the code classified it as a "this should never happen!" error, and then it happened. The code didn't classify it as a "flight plan has bad data" error or a "flight plan data is OK but we don't support it yet" error.
If a "this should never happen!" error occurs, then you don't know what's wrong with the system or how bad or far-reaching the effects are. Maybe it's like what happened here and you could have continued. Or maybe you're getting the error because the software has a catastrophic new bug that will silently corrupt all the other flight plans and get people killed. You don't know whether it is or isn't safe to continue, so you stop.
If a "this should never happen!" error occurs, then you don't know what's wrong with the system or how bad or far-reaching the effects are. Maybe it's like what happened here and you could have continued. Or maybe you're getting the error because the software has a catastrophic new bug that will silently corrupt all the other flight plans and get people killed. You don't know whether it is or isn't safe to continue, so you stop.