Why are alternative solutions not welcome?
They are. Note the text:
You have presented an alternative solution, but haven't reviewed the code.
The problem is not that it shows an alternative solution but that it doesn't review the code. Every answer should make at least one observation about the code posted in the question.
To help explain what that means, there is the next sentence.
Please explain your reasoning (how your solution works and why it is better than the original) so that the author and other readers can learn from your thought process.
The "why it is better than the original" can be the one observation about the original code.
If you think that a revised wording of the message would make this clearer, you are welcome to propose one.
We like answers that propose alternative solutions. We especially like them if they are nifty ones that we might use for something someday. What we don't want to see is the asker being buried in alternatives without any guidance as to what makes them better. This is frustrating for the asker and can lead to problems. Some alternative solutions are specific to particular conditions of the problem.
Another problem is that an alternative solution may not be better. If you can't explain why an alternative solution is better than the original, then maybe it isn't.
In that particular case, note that it wasn't actually an alternative solution. It was part of an alternative solution. That would have been much more obvious with some added explanation for context.