The reasoning behind this is related to meta and balancing.
Balancing is obvious - less choices is easier for balancing.
For the metagame however the optimal number of item slots depends mainly on the itemization (and its balancing) in the game. The more (after certain value X) we increase the number of item slots (X → X + 1 → X + 2 → X + 100 = Y) the bigger the character identity and unique builds. The more we shrink their numbers the more meta builds per meta-tier we’ll witness.
Let’s say tier 1 meta composition for 4s is only one. A potential Y scenario will pretty much render tier 2/3/4 composition again a single one, while a small X would serve for much diversity in tier 2/3/4.
So, the real problem for them is to find that optimal X serving best for what they want to achieve. And this is done through Math depending on the itemization, its balancing and their goals (meta tiers vs character identity).