Because it's been shown that the Crucifixion actually took place earlier in the week, but the later exegetes, unfamiliar with the Hebrew calendar, assumed the sabbath being spoken of was the weekly sabbath, and not a High Day in the calendar.
Because no-one can evidence an extra-Biblical idiomatic usage of three days/three nights, that does not in itself constitute a proof that it was not used idiomatically, and especially when Hebrew and the languages of antiquity were clearly idiomatic.
The argument put forward, on that basis, is that the phrase does not absolutely and definitively mean a 72-hour period.
That Tradition continues its observance of the Easter Triduum is because it's observed in spirit.
if we re-organised the calendar to cover the probably more accurate dating that puts the crucifixion on a Wednesday, then the three day prophecy is covered absolutely, but the spirit if the occasion would not be changed.
The Easter-observant Christians – Catholic, Reformed, Orthodox, etc., would say, "OK, so what? The point is He was crucified for us, and was raised from the dead – that's all that matters."
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I've answered your three day problem:
1) He was crucified most probably on a Wednesday, possibly a Thursday.
2) Hebrew is an idiomatic language so we need not necessarily read 'three days three nights' as absolutely 72 hours. The number three, among others, has special significance. They were not bound to literalism quite as we can be.
3) Tradition continues many beliefs and practices we know to be not-quite what is assumed – we ascribe the Gospels to four named persons, but we accept the authorship is anonymous.
In the larger scheme of things ...
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This three-day thing seems a much bigger deal for you, than for anyone else.