Open sourcing the algorithm would be such a huge boon to those wishing to game it. Those with the sufficient compute resources could run billions of variations of Twitter activity to perfect their messaging to get it trending. It'd be an arms race for those with enough money to keep up. Any chance of anything ever making it into trending in an organic way would be lost.
For transparency, it might be nice for Twitter to continually release their algorithm from a certain time period ago, say 6 months
Unfortunately it wouldn't even look anything like that. "The algorithm" is probably a bunch of ML with some shitty product driven hacks on top, and those hacks will be trivial to game. Things like "if follower_count > X: spam_model_weight *= 0.1"
For transparency, it might be nice for Twitter to continually release their algorithm from a certain time period ago, say 6 months