Length may be part of it, but personally, there are a lot more reasons.
Separating articles on words and on styles makes them easier to list and categorize.
Learners are likely looking for the word itself, not workarounds. It's easy to underestimate how confusing or intimidating these articles might already be for people new to Toki Pona, so we should keep the usage sections focused on the main concepts so it's easy to follow along, scan for specific details on repeat visits, etc.
pi is unfortunately a core word, and one of the few standard sentence-structuring particles. Providing good information on these words' usage is probably a main goal and use case for the wiki. There's a distinction between correcting common mistakes (which furthers that goal, providing information on how not to misuse these words), and this merge (which seems like the opposite, mixing in information on how to disuse these words). Even if this article weren't as long, that fact would remain.
The merged article could also seem overly biased, giving pin't undue weight.