Keeping in contact with mobile phones can get expensive - and if you work with different people, you might not always have your colleagues number.
Network coverage can be patchy at the best of times in some rural areas and cannot always be relied on.
With multi-vehicle operations, harvests etc, there really is no substitute for radio communications, mainly because what is said is heard simultaneously by everyone on that channel, making tasks like this a LOT easier.
The mobile phone "handsfree" law affects any vehicle on the public road, including agricultural machines, whereas CB is not affected by this law at all.
You might be able to get away with using mobiles for quick "yes", "no" "meet you in the top field" type snippets of conversation, but one of the best things about CB is that people can chat together, work through problems and discuss what they are doing, and it's this increase in effectiveness, efficiency and more often than not - friendship, that really only CB can offer.