At present, John Cena is busy filming Fast & Furious 9 alongside the likes of Vin Diesel, Jordana Brewster, and Michelle Rodriguez. This could be the biggest reason why Cena might not appear at the upcoming Raw Reunion show.
Hollywood filming schedules are brutal, as evident from The Rock's absence from WWE for around 7 long years. Cena's schedule might not allow him to commit to WWE for even a single appearance at this point in time.
It's no secret that Cena is now a part of the hit franchise "The Fast & The Furious", with its ninth installment coming out next year. While he could be busy filming the same, Cena has a great opportunity to promote his movie in front of a large crowd, plus the millions watching at home by making an appearance at Raw Reunion. A short segment could see him thanking the WWE Universe for their support, and putting in a line about the upcoming movie.