An adverbial clause of reason has its verb in the indicative, unless the reason is an alleged one.
Haec aetas moribus apta mels ... quia cultus adest.
Ovid Ars amatoria 3.112S.
This age is suited to my character ... because now there is refinement.
Noctu ambulabat in publico Themistocles, quod somnum capere non posset. *Cicero Tusculan Disputations 4.44
Themistocles used to walk in public at night, on the grounds that he could not get to sleep, (posset is subjunctive because the reason given is alleged and not necessarily true)
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