Mostly because they weren't designed to be parallel. Lots and lots of reasons, but no idea which might apply to your situation due to a lack of detail in your question.
The voltages may have different phase angles. They may come from different sources and paralleling the incomers could cause undesirable circulating currents between the sources.
emillar:
There may be non-technical reasons, such as safety. As davidbeach indicated you haven't provided enough detail about your situation to respond properly.
with 11kv you need to check your sequence and connections, Your rotation may be the same on both supplies but the sequence may be out. ie. A-B-C and B-C-A on the other feeder. trying to connect those and you would have an issue.
Are the 11kv Supplies from the same source ? Not a generator and utility supply?
fault levels may be significantly changed by paralleling up two supplies enought that it would exceed that could be safely handled by the fitted protection.
It is however common to allow parallel operation during switching to prevent loss of power but this is done carefully and under the instruction of the supply company and generally involves a very short duration of time that the two supplies are run in unison.
