[Celjabba]

In 3e , it was ST - fp and HT - hp. (with the reverse a common houserule and an official alternative in Compendium I)
4e switched those:
ST = mass = hp
HT = fitness = fp

Basing hp on HT make some sense but also have a double effect regarding injuries: low HT is double bad; high HT is double good.
And you need split HT rules for small and huge creatures. (unless you want all rat dying of sickness within a day, while near-immortal 500 years old elephants swim in lake Anthrax).
Since most creatures get more hp and more st as they get bigger, linking the 2 stats avoided split stats in most cases.

Basing fp on ST had a lot of small consequences:
-it make some build difficult (like the above marathon runner),
-small critters couldn't run across a field without passing out from exhaustion.
A t-rex would only need to stop halfway to Asia. Likewise, this was solved with split stats.
-all 3e mages were bodybuilder, (if not centaur).

Edit : Gollum said it better.