Global food and local housing was a thing in WoM. WoM didn't go over well, so instead of building FE as an expansion to WoM, they basically built a full new game. With FE, they decided to make it so that the prestige of an empire determined how rapidly it grew. Presumably, food was not chosen to be the determinant in growth rate at that point because Stardock felt that the prestige-based growth mechanic should be dominant. At that point, they then needed to decide how much infrastructure they wanted the player to have to build to grow a city - do you need housing, food, or both? The choice for FE was that the player only needed to concern themselves with the local food supply.
However, the prestige-growth mechanic was one of the less popular mechanics in FE because it meant that your entire faction's growth per turn was roughly equal to your prestige, but reduced because all cities gained an equal fraction of the prestige growth even if the city could not support additional population and this additional population simply disappeared rather than being given to a city that could support more people. There were also a handful of structures and a spell or two that increased population growth, mostly available to Kingdom factions, which gave the Kingdoms a significant leg up in terms of developing reasonably high-level cities; these structures and spells were highly desirable because they boosted growth rate regardless of faction prestige.
Because prestige-growth lacked popularity, at least as implemented, with the forums, prestige-growth was removed in LH. This, however, left a vacuum - there was a need for a base growth mechanic to cover the hole left by removing prestige. This is the genesis of the food surplus growth rate found in LH (which, I might add, is not the same as could be found in WoM - in WoM, food essentially served as a global cap and housing served as a local cap, and the growth rate was more or less fixed, if I recall correctly).
So, to make a long story short, as I see it global food + housing (for global/local population caps, respectively) was converted to food as a local population cap + prestige-based growth in FE because Stardock felt that the way population was handled in WoM was something that FE could improve upon. Then, because the improvement settled upon for FE wasn't as good as it perhaps could have been, or wasn't all that they had wanted it to be, the prestige-growth mechanic was removed in LH and the food-growth mechanic was added.
Some side notes:
- As implemented in WoM, food served as a cap on the total housing structures you could build throughout your empire. If I recall correctly, it had no impact on population growth aside from that without X units of food you could not build Y units of housing, and thus could not further increase the population of a city beyond its current housing limit if you were out of food. A somewhat amusing interpretation of the game mechanics could be that houses eat food, and that up to X people can be supported by a single house.
- Food surpluses as a source of population growth only showed up in LH. FE used (your faction's prestige)/(the number of cities in your empire) as the base growth for a faction, with some structures and a spell or two providing a flat +1 or 2 growth (which stacked). These structures and spells have carried over to LH and some additional ones were added; I don't think any were removed. I don't recall WoM's exact mechanics particularly clearly, but I think it was something along the lines of each city getting something like +1 growth per turn, plus bonuses from some structures and certain spells.
- There used to be a materials bonus for clearing forests; this can be seen in the XML, but, as with road levels, has not been present since WoM.
- Population, not improvements, was once the source of your empire's wealth. That went the way of the dodo with many of WoM's other mechanics when FE came out.
