“até porque” (or “até mesmo porque”) is often used colloquially in ptBR and its nearest synonym would be “inclusive porque”. To me, it's a set phrase and idiom ("até" has one meaning, "porque" has another meaning, and the two words together work with a different meaning.) and as Artefacto mentioned in his answer, the phrase usually introduces an afterthought and is also common in ptPT.
I was trying to translate it into English but couldn’t find a word or phrase that expressed exactly the same. I thought of quite a few phrases but didn’t like any. “Even because” sounds like a poor choice to me. “Also because” isn't a set phrase and doesn’t seem to convey exactly the same.
Examples of "até porque" and "até mesmo porque" in Portuguese:
- Eu já disse que teu livro não está comigo. Não levei comigo, podes ter certeza. Até porque eu não tenho nenhum interesse em filosofia.
- Não acredito que tenha sido o João. Ele não faria isso. Até mesmo porque ele também foi prejudicado.
How can we convey exactly the same meaning in English?