Hi
I think it has to do with the flow of the trigger logic. If you have a trigger repeating within a block that repeats, there is an endless loop, so the block of triggers does not come to a natural end. By deactivating the loop, the trigger logic ends naturally, and the block can then be closed and repeated.
The V6 manual offers a good explanation:
"Using a "close block" trigger you can set the conditions under which a block has to be repeated. The very words "close block" mean that all the triggers in the block have been duly executed (or don't need to be executed any more), and it should be considered "closed", and needs to be "reopened" (that is, repeated). Until a block is not closed, it won't be repeated."