Its just that a laptop costs more money, bang-wise for the buck. You generally can have only one internal harddrive (most NLEs give you better performance if video files are on a drive that is not the system drive) and laptops are not as upgradable.
Having said all that, it certainly is more convenient to lug a laptop than a full-size tower case and monitor.
As for going Mac, software, hardware and accessories are generally more plentiful and less expensive for PCs.