I would take it as ablative showing cause, so that it just anticipates the causal quod that follows. Therefore, eo, quod means something like 'for this [very] reason, namely because' or simply 'because.'
One classical example of this use of eo quod is found in Cicero, Philippics 13.23:
'<A senatu> iudicatum hostem populi Romani Dolabellam eo quod sicarium occiderit, et videri cariorem rei publicae filium scurrae quam C. Caesarem, patriae parentem, ingemiscendum est.'
'That Dolabella should at this time have been pronounced a public enemy because he has slain an assassin; and that the son of a buffoon should appear dearer to the Roman people than Caius Caesar, the father of his country, are circumstances to be lamented.'
[Translation by C. D. Yonge, from the Perseus website]