This might be because of the size of the mobi file. Amazon has a limit of 50MB for sending to Kindle, so unfortunately we can't offer this option for larger ebooks.
Instead, you can transfer larger ebooks to your Kindle by downloading them to your computer, connecting your Kindle to your computer with a USB cord, and moving the file into the Kindle's documents folder. Let me know if you have questions, and you can find more information on Amazon's help page.
I hope this helps!
Customer Service Representative