You must understand / You will appreciate that I have had no time to prepare a speech.
[infml] The reason why he left was the lack of prospects. / He left because of / on account of the lack of prospects / because there were no prospects of promotion.
The reason for his departure was the lack of prospects.
In view of / In the light of / Given his uncooperative attitude I am not giving him any more work.
The delay in delivery is due to circumstances beyond our control.

[infml] - nieformalnie