Learn Dutch: Ik fiets niet omdat het verkeer gevaarlijk is. - I am not cycling because the traffic is dangerous.

The word omdat (because) introduces a dependent clause. In Dutch, this type of connecting word forces the verb (is) to move all the way to the end of its part of the sentence.

Could I use want instead of omdat to say "because"?

Yes, but it would change the word order! If you use want, the verb stays in its normal position: ...want het verkeer is gevaarlijk. Both words mean "because", but only omdat pushes the verb to the end.

Why does niet come right after fiets?

In the first part of the sentence (Ik fiets niet), you are negating the entire action of cycling. When negating a whole verb action without any specific object, niet simply goes at the end of that clause.

Why doesn't the adjective gevaarlijk get an extra -e at the end here?

Because it comes after the verb is to describe the traffic. You only add an -e to an adjective when it is placed directly in front of the noun, such as in het gevaarlijke verkeer.