Because that coincided with the beginning of the 365-day civil year
in 1270 BC, during the reign of Rameses II, Belmonte and Shaltout suggest that the ancient architects designed the temple to line up with the rising Sun at the beginning of each of the year's other two seasons, in October and in February.
And yet, the heliacal rising of Sirius that occurred on or about a certain specific date every year would actually move forward one day every four years with the clerical 365-day calendar of the Egyptians, so that by the time of the post-Alexandrian pharaohs it was deemed so badly in need of reform that, once and for all, the ecclesiastic year was decreed that it should conform with the civil year
. So it was that the Canopus Decree was issued in the days of Ptolemy III Euergetes (238 BCE) to correct this discrepancy.