I'd assume it's simply because you get a more consistent and correct wage when you're working based on hourly conditions as 4 weeks will always be 4 weeks but a month varies in length both from a calander point of view and from the amount of working days you'll actually hit.
This will allow whoevers processing your timesheet to enter the same details every 4 weeks with the exception of those who have taken unpaid leave or worked overtime, which can be noted in a short list, assuming you work a consistent amount of time per week making it easier for them to do this, assuming it's being done / checked manually.
If you pay monthly and actually pay based on hour, you should be giving your staff a different amount every month based on an incosistent amount of works worked, but obviously if they're salaried you can happily work out their hourly based on the annual hours and then will need to do some math to square up with them when they quit on you.
In the end it doesn't matter. You should get paid what you're owed under any system and people who say "I like that I get paid twice in december" are people who are obviously crap at budgeting.