Why is "town"not accepted?
The word "and" seems to be required, but it needn't be. "This is a big important city." should be sufficient.
That happened to me for both this question and another one in this lesson. I reported both of them.
why is large city not accepted?
It should be. Large and big are both fine. You can report it if it comes up again. I think they just used "big" rather than large in all these sentences originally, and are gradually adding "large" as an accepted answer when people report it.
Large would be the preferred word for a native English speaker. The use of big would be slightly odd
I'm a native English speaker and I don't find anything odd about 'big' in this sentence. Both words sound fine to me.