Outside of Israel we say ...ברוך ד' לעולם after השכיבנו by Maariv. Why is this part of tefilah not said in Israel (or: why is this said outside Israel)?
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Ashkenazim in Eretz Yisroel generally follow the customs of the GR"A. One of his customs was omitting Boruch HaShem Leolam as an unnecessary interruption between the blessings following the Shema and the Shemone Esre.
I would cite Siddur HaGR"A as a source except that I don't have a copy to confirm.
As for why it is said, this related question might be a good place to start.