Sorry, I'd only just seen your message.
Why should it be necessary to ise HTTP in order to access IRC
Because the browser has been around half as long and has had a billion more advancements. This is a small part of an effort to provide server administrators with the option to delegate their registration process to a website..
This could be for a number of reasons, including things like the host wanting to truly customise the registration process and have it exactly how they like. You can very easily, with much user friendliness, do things like upload a profile photo which, if their client supports it, can also be their IRC avatar (dw METADATA spec rewrite underway). Also, whenever anyone is told to sign up to anywhere outside of IRC circles, everyone is very used to "we sign up on websites".
Aside from this, there is Anope services which provides an optional webpanel for user registration, and DALEK which provides straight up full wordpress integration, where the registration form for the network lives.