Speaking as someone with an outsider's perspective, I think the reason the Cell arc seems to be the most loved in the USA is because it was the first arc to be consistently produced for its English dub. With the Saiyan and Frieza arcs, Goku went through three different voice actors, and the entire voice cast was changed by the time he arrived on Namek. The soundtrack shifted from Shuki Levy's score to Bruce Falcouner's, and the censorship standards shifted. The actors took a while to settle into their roles, so by the time of the Cell arc the production had improved. The arc itself has some of the most questionable writing and character moments of the entire series which really undermines its excellent emotional moments.
As for us in the UK, we watched Funimation's dub until the introduction of Trunks, then we were shifted into watching the Ocean Dub for the remainder of the Cell arc. As such, I personally don't hold the same love for the Cell arc and think that the Frieza arc is the absolute best arc of Dragon Ball.
I briefly checked through episodes 7-11 and 11-16 of Kai to refresh my memory. The guys have chosen a great pace with 5 episodes for each session, and they'll have some nail-biting cliffhangers to endure too.
I'm gonna answer the questions on this list which aren't addressed in either Z or Kai due to the fact that they were addressed in the original Dragon Ball and the show expects you to have watched that.
1. Yamcha is a desert bandit famous for being the buttmonkey of Dragon Ball. He has been Bulma's boyfriend since the end of the first arc of Dragon Ball and he trained under Master Roshi like Goku and Krillin did. He plays baseball to earn a living.
2. Most of the characters, including Goku, learned how to fly during the 23rd Tenkaichi Budokai tournament, which was about 5 years before the start of DBZ Kai.
3. The Earth presented in Dragon Ball is a fantasy land full of demi-humans and dinosaurs, but also technologically advanced cities and capsule technology. It's one of those things which makes the series stand out, but has less relevance as the show goes on. Actually, what constitutes a "human" in Dragon Ball has been the subject of some discussion during the Goku Black arc of Super.
4. Goku, Master Roshi and Krillin seem immune to bullets as they've taken a lot of shots from Bulma and Launch in Dragon Ball.
@nicksmi56: The question you should ask yourself is why would they pick One Piece?
Dragon Ball may be older, but it's been gaining popularity again in the light of Kai, the recent movies, Super and FighterZ. It's only natural that they show some interest in its recent resurgence of popularity.