For #1, branding. They made a big deal of the 50th bowl and not that many people know L is 50. Also, the connotation of L as Lame or Loser may have played role. I belive they return to LI next year.
For #2, interesting question. I believe it’s that the full NFL product came after the merger. As a fan of a pre-merger team, a lot of important history is left behind. The juggernaut of professional football happened in the Super Bowl era...really starting probably at Superbowl III when it became a true national phenomenon. Superbowl I was somewhat of an afterthought as the teams were more focused on winning their own league championship, especially on the Packers’ side. Once the merger took hold and the AFL team won their first, I think it changed everything in the sport. I was not alive, but from learning about the history of the Super Bowl era, that’s my sense. They had a more complete product and a wider audience once the merger took hold and both leagues looked like legit contenders.
It’s 50 because this the 50th Super Bowl aka the NFL-AFL Championship in its first 2 editions. The game was first played in 1967. The AFL was not founded until 1961.
We’ll have a few laughs a decade from now with Super Bowl 59 ... heh-heh!