I can answer why one got reboots and sequels:
Hot Pursuit is a very clearly defined gimmick: racing with cops. It was a thing since first game, The Need For Speed, and only went missing in NFS2, then was, again, cut in Underground, but every time cops reappeared in the series, that was heavily marketed, so Most Wanted and Hot Pursuit subtitles were re-re-used.
High Stakes mode was a multiplayer-only gimmick similar to a pink slip race in later games, you bet your car against opponent (it's erased from memory card in PS1), and it... felt like an afterthought... BUT because it required distinct cars "owned" in your garage, not just unlocked, they had to slap a career mode on. This was a meat of the game, GT-lite but with real car damage.
Now, racing-RPGs got really huge at the time thanks to Gran Turismo, THE top selling PlayStation game. Need For Speed tried to follow that simcade formula many times, but didn't reuse "High Stakes" subtitle simply because it's ehhhhhh.... hard to sell to people? Don't mean financially, I never cared for the mode itself in 4 and it was my second-fav racer after GT2.
MANY later NFSes have a career and car customization, but they were split between "street" racers like Undegrounds, Most Wanteds, Carbon, Heat etc., and circuit ones like Porsche (really good career, physics and tuning. despite me not caring for that brand), Carbon and ProStreet. So the new titles focused on vibes instead of "you can buy, sell, and bet your car" that was NFS4's main gimmick.
Hot Pursuit is always used for the ones where you can't customize cars, right? The more arcadish games. So people know what they're getting... my fav part was Chase H.Q.-like mode where you play as a cop and RAM into other cars, we need more Destruction Derby! xD
p.s.
Playing NFS5: PU is worth it, just note that PS1 and PC versions are very different, I liked both and I agree with you on one-make racers and Porsches. If you liked Underground and want more Japanese cars, Street Racing Syndicate is pretty good. It's on Steam.