It is not a case of BE + of, but of BE + of + (adjective) noun
This question is of vital importance = it is vitally important.
This work is of dubious quality = The quality of this work is dubious.
'For' does not mean 'because' here.
This car is too expensive for me. (I cannot afford it)
This car is too expensive for me to run. (I cannot afford to run it).