Why is there no guiding guardian for one whom Allah has led astray, despite the presence of numerous preachers and religious sermons?

There is no contradiction between "for he whom Allah sends astray, for him you will find no protecting guide" and the existence of callers to Allah. Guidance is of two types: guidance of divine grace (Tawfiq) and inspiration (Ilham), which is exclusive to Allah Almighty. No one can be guided without it. The second type is guidance of instruction (Irshad) and clarification (Bayan), which is carried out by prophets and callers to Allah. Whoever has been decreed by Allah to receive the guidance of divine grace benefits from the call of these callers; otherwise, they do not.