How should I parse the part that comes after 許さない:
その形容から打突こそ主体と思われるが、槍｛やり｝の基本戦術は払いにある (1)。長さに物を言わせた広範囲の薙｛な｝ぎ払いは、もとより身を引いて躱｛かわ｝す、などという防御を許さないからだ (2)。
Feels like it makes the sentence flow a bit better with sentence topic but what is it, exactly? から_だ? から used to affirm what's stated in (1) + copula for emphasis?
「～～からだ」＝「～～からである」＝「～～からです」
"から used to affirm what's stated in (1) + copula for emphasis?"
Kind of but not quite. It affirms a prior statement by explaining the reason and/or logic behind the content of the prior statement.
The copula is not there for emphasis. It is just needed there for the grammar reason. In informal speech, it can be dropped and you could end a sentence with 「から」.
Statement #2 explains why the sweeping is the core of the spear fighting instead of the attacks.
