ため[に]、 ...・...ためだ (cause / reason)
Usage
(S plain form / いA / なA -な / Nの) + (ため(に)+ ためだ). In [Xために, Y], X is a cause or reason, and Y is what happened. This is used in writing and is more formal ten から and ので. When the result is stated first, and the cause or reason is given as a predicate, the construction becomes [Y(の)はXためだ].
Examples
航空が使えなくなったのは、大雪が降ったためです。
The reason why the airport became unusable was that a lot of snow fell