I think this is best handled by contacting the lecturer, but here are four reasons.
A Kain0_0 remarks, your solution is recursive, while his isn't.
Didactic considerations behind this might be:
Recursion is hard for some students. Perhaps it hasn't been taught at this point in the course, or is considered an unnecessary burden.
Most BFS implementations in ...
The maximum degree of a vertex is $25n$, since this is the number of words which can be obtained by changing a single character. I'm not sure how you get to $26^n$.
You are writing "$4$ is more than $O(n)$" (where $n=2$). This is completely meaningless, since $O(n)$ stands for a function bounded by $Cn$ for some unknown constant $C$. If you don't know the ...
I recommend you to take the same approach as I did to draw my conclusions: take a graphical approach to see the differences and drawbacks.
For that, do the following:
Comment out all print lines except print("Frontier:", frontier)
Then do python program.py > version1.txt, modify your program such that it represents verison 2, and then do python program....