Dale Sams wrote:
Between TOS and TNG, the ability to timetravel at will seems to have disapeered all together, not just an edict not to do it, but to do it at all.
Did lit ever explain this as perhaps some cosmic race stepping in and saying "We'll just change physics. This is too dangerous".
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The novel
DTI: Forgotten History goes into detail about this, and explains why so few people know of the technique (and even fewer are able to use it) in the Next Generation era.
In short...
Of course, the real life reason is that an easily achievable undo button robs any story of consequence or tension, so they just pretended it didn't exist.