The plane's delay was due to mechanical problems.
The plane's delay was because of mechanical problems.
The plane's delay was due to mechanical problems.
The plane's delay was because of mechanical problems.
If X causes Y, then a very natural phrasing is "Y is due to X." The phrasing "Y is because of X" is unidiomatic and is unnecessarily wordy. "Due to" is two short syllables whereas "because of" is three syllables.