The primary purpose of each group is to carry its message to the alcoholic who still suffers. Leaving aside the fraught question of what 'its' message is, as even asking this can split the group down the middle, let's assume that the group's message is the AA message. What is the AA message?
According to the third edition of the Big Book, it is the basic text (the first 164 pages).
(NB the fourth edition omits this reference, but that's another discussion.)
The purpose of the main speaker of any AA meeting is therefore to carry the message contained in the first 164 pages, and the only way to do that is to have had the experience of taking the steps as outlined there.
Taking the steps in other ways may be very good, may have brought about recovery from alcoholism, may have done all sorts of good things, but this is not the AA message; it is another message. The instructions on how to work the steps, as a single, coherent package, are contained in the Big Book and nowhere else. The Twelve Steps and Twelve Traditions provide a commentary and additional tips on the main instructions but do not replace them.
It should be clear, therefore, that to qualify to pitch as the main speaker at an AA meeting one must have experience of all twelve steps through the Big Book, and be prepared to have this as the main focus of one's talk.
Good speakers will help newcomers or others to identify as alcoholics, using the two main features described up to page 44 (craving triggered by the first drink; inability to stay stopped), and then will discuss the solution: the steps, fellowship, service to others, God; and will go on to describe the results and benefits. There is no other suitable topic.
An effective secretary, chair-finder, etc. (the terminology changes around the world) will therefore listen out for individuals who are already speaking along these lines, and pick only those individuals to speak.