1
There was a traffic jam because the snow.
There was a traffic jam because of the snow.
Use because of before a noun or noun phrase.
2
I haven’t called because of I’ve been very busy lately.
I haven’t called because I’ve been very busy lately.
Use because before a clause (subject + verb).
Compare:
- There was a traffic jam because it snowed heavily.
- There was a traffic jam because of the heavy snow.