What prompted this change when it's clear several newer Champions will get the short end of the stick?
Base stats are incredibly uniform in most cases. Max HP, AD per level, Movement Speed, armor, magic resistance: - none of those very by much from champion to champion. For most mental math purposes, you can generally assume a champion has:
500+85=2000 HP
6.8+0.66=18 HP/5
275+45=1000 MP
7.25+0.55=16.7 MP/5
55+3.2=110 AD
0.66+2.5=1.0 AS
18+3.5=75 Armor
30+0=30 MR (ranged)
30+1.25 = 52.5 MR (melee)
For most intents and purposes, doing stuff like Sheen/Trinity/Gauntlet off of "base AD" means "Scales with your level".
Same deal with this.
Obviously there will be a few outlier cases like Riven when it comes to base stats. For those instances - it's important to note Riven also isn't putting a Deathcap, Hurricane, or Archangel's staff to use. Not every item is meant to be useful on every champion, and Riven isn't the kind of fighter that actually wants to rely on HP/5 over Life Steal for her health sustain.
Also - HP/5 and MP/5 are super crazy awesome in the lanning phase. Tiny amounts can make a big difference, as poking an enemy out can be a 3 minute endeavor.
HP/5 and MP/5 are not amazing stats end-game though. Being useful over a ~3 minute span of time is not useful when big fights happen within ~10 second windows late game.
Allowing HP/5 to scale with levels will allow a gradual transition to where it needs to be late-game, without sacrificing it's late game potency (what it currently does) or without making it brutally OP during lane so it's useful late.