The Pope told us why:
After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry. I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering. However, in today’s world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the barque of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me.
Any speculation beyond that is unnecessary at best, the sin of rash judgment at worst. Conspiracy theories aren’t going to offer useful explanations.
The one thing that is certain is that the Pope made this decision freely without coercion or being under constraints. That’s a necessary prerequisite for the Pope to step down. He can’t just step down because times get rough and he wants out of the responsibility.