They would have made this decision by looking at the amount of people who still use 32bit systems VS everyone on 64bit systems today, they would have that kind of info to know mostly what percentage of the playerbase is still using 32bit systems (Which I imagine is a far lower number than it was in 2014) and be able to say:
"Well 5% of players are on 32bit systems today, but optimizing and developing for it is adding x hours to the development time of packs, expansions and updates on top of limiting the possible updates and changes that could be done for the game. We just can't justify that amount of time dedicated to an architecture that is that outdated."
I don't actually know the specifics of course and that is just an example, but it is often how these sorts of things unfold with game development.
Especially since this mostly affects the complexity of changes they can make and what can be added to the game. The larger and more content filled Sims becomes the more hardware it demands. It's very straight forward, more stuff = more requirements and 32bit systems can only use a very small amount of ram by today's standards, about 4gb when a lot of regular PCs have 8gb or more. Remember 64bit systems have existed since the early 2000s at this point and have been standard for at least 5+ years, if not longer.
It actually makes me wonder if a lot of my gripes over the Sims 4 being weirdly shallow in parts is due to the fact they have been developing the game with 32bit systems in mind and working within those limitations when they could really stretch their legs out only working on 64bit systems.
I guess I won't know for sure until sims 5 someday. The Sims 4 isn't going to change much at this point, the change will mostly be to streamline their workload and have the option to possibly do more intricate things with the game.
I can understand the frustration but there really is no way around this without simply restricting the game from a development standpoint across the board, it'd be like asking a fully grown adult to try and sleep in an old baby's cot because it is the only spare "bed" you have in your house.
It's just not viable. It's a different time, 64bit isn't some new thing. There were 64bit versions of Windows XP, if you really want to put this into perspective.