Yeshua died in 30 CE. The Temple still existed till 70 CE, 40 years after his death. That’s ONE generation from Yeshua's death to the end/destruction of the Temple for YHWH’s people to come to understand what He did for humanity by sending His Divine Messiah to redeem us.
Most Jews to this day still do NOT believe that Yeshua’s death served as our redemption; all they know is that since the destruction of the Temple, they have no way of properly sacrificing animals, and have arbitrarily decided that a blood sin sacrifice is now unnecessary as God only requires prayer and supplication (which is NOT what the Torah says!)….
Anyway, the number forty appears many times in the Bible, usually designating a time of radical transition or transformation. (For instance, the Israelites had to wander in the desert for 40 years, when the first generation of the promise finally got to enter the Promised land.)
What could be more "transforming" than the substitution of the nephesh/soul of Yeshua, so that within one generation, all who accepted this gift could be eternally saved and retained in YHWH's favor by continued obedience?