Without knowing all ins and outs theres few reasons.

1) Some countries simply dont take electric signatures (you know, html checkbox that says "yes") as acceptable. You have to fax your sig or something like that. Setting up that might cost too much money or hassle (you would actually *need* customer service people or more of them instead of this automated service that it is now)
Imagine getting thousands at first weeks and hundreds afterwards of faxes everyday where you have to locate who made the sig (people usually forget to fill stuff) then go to their account info and enable it and then save the signature for future reference and then send copy one way or another of the contract back...that thousands of times a day? Needs more resources and people.

2) Some countries (specially few in EU) are against contracts that give "freebies". For example long time ago in my country it was not allowed to give phone with new contracts, it "interfered with the competition" or something.
So basically attaching iPhone into operator was not allowed at all. Nowdays you can here but this type of law remains in some countries..

3) Kinda same as 2. For example I believe you still cant RaF in some countries because its illegal to get someone into buying something and get "something out of it". Not related to annual I guess but it can be..I dont know.

Or simply you are not allowed to bind someone into long contracts they cant get out of AT ALL..