Background: Edward Leo Peter McMahon Jr. (March 6, 1923 - June 23, 2009) was an American announcer, game show host, comedian, actor and singer. McMahon and Johnny Carson began their long association in their first TV series, the ABC game show Who Do You Trust?, running from 1957 to 1962. Then afterwards, McMahon would make his famous thirty-year mark as Carson's sidekick, announcer and second banana on NBC's highly successfully The Tonight Show Starring Johnny Carson from 1962 to 1992.
Context: The pair joined The Tonight Show Starring Johnny Carson on October 1, 1962 on NBC. He describes what happened when the pair first met, the whole meeting being "... about as exciting as watching a traffic light change". For almost 30 years, McMahon introduced the show with a drawn-out "Heeere's Johnny!" His booming voice and constant laughter alongside the "King of Late Night" earned McMahon the nickname the "Human Laugh Track" and "Toymaker to the King". As part of the introductory patter to The Tonight Show, McMahon would state his name out loud, pronouncing it as , but neither long-time cohort Carson nor anyone else who interviewed him ever seemed to pick up on that subtlety, usually pronouncing his name .  Aside from his co-hosting duties, it also fell upon McMahon during the early years of Carson's tenure (when the show ran 105 minutes) to host the first fifteen minutes of Tonight, which did not air nationally. McMahon also served as guest host on at least one occasion, substituting for Carson during a week of programs that aired between July 29 and August 2, 1963, and again for two nights in October 1963. McMahon served as a counter to the notoriously shy Carson. Nonetheless, McMahon once told an interviewer that after his many decades as an emcee, he would still get "butterflies" in his stomach every time he would walk onto a stage and would use that nervousness as a source of energy.  His famous opening line, "Heeere's Johnny!", was used in the 1980 horror film The Shining by the character Jack Torrance (played by Jack Nicholson) as he goes after his wife and child with an axe. He did in-program commercials for many sponsors of The Tonight Show, most notably Budweiser beer and Alpo dog food, and also did commercials for them that ran on other programs.
Question: Why did it fall upon McMahons shoulders?

Answer:
substituting for Carson during a week of programs that aired between July 29 and August 2, 1963, and again for two nights in October 1963.