Why don’t you drop the fan translation when the official translation catches up? We will wait years without a fan translation.
Assuming you are asking this in good faith: usually it's to support the author.
By continuing to release a scanlation when there is an official translation, you directly compete with and eat into official sales, because most people would rather access something for free instead of paying, even if the free alternative is of lesser quality.
Most scanlators that drop licensed series are in a camp that believes a scanlation only serves its purpose so long as there is no official alternative available. Once an official alternative exists, they step away and let that take over. The hope is that this fosters the official industry and leads to more works being translated with high quality. And if sales increase, companies can also provide better services like simulpubs and the like.
Remember, if nobody pays for the manga, there is no manga.
One other reason is just time. Why focus your time and energy on a scanlation when a legal, usually better alternative will come out later anyway. You might as well spend your time on scanlating something that doesn't already have a translation in the works. That's just being efficient with your life.
I know for many readers none of this matters, they just want their fix, but it is what it is.