Because Alexandria, Egypt was a thriving city with dozens of ancient monasteries and libraries. Many early Christians went there in the first few centuries AD and stored treasure troves of copies of the New Testament books. This was destroyed by the Roman emperor Diocletian during the terrible persecution of 300 AD, but fortunately many scholars had snuck New Testament books into hiding before the Romans burned them all. As a result, any manuscript pre-dating 300 is referred to as an "Alexandrian scripture."
John is indeed the most copied and also the earliest of all New Testament fragments...some have even been found as early as 90 AD (within 30 years of the original). Word didn't travel as fast in the ancient world as it does today so that copies had circulated as far as Egypt by that early is very strong support for historical Christianity...I find it incredulous that so many skeptics claim the gospels were written after 100 AD when all the evidence directly contradicts that.
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>> “John never went to Egypt, yet more fragments were found there than any place else.” <<
Try not to forget that the place where the gospels were written was sacked by Rome in 68 AD. Little can be found of parchments or Papiri after a city is burned and smashed.
The most complete copies of the NT writings were found in Spain, far from their origin.
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Agree that the dating of John’s gospel could not possibly be later than 100AD. In fact, I submit it could not have been written after 70AD. The Roman general Titus in 70AD completely destroyed Jerusalem such that one stone was not standing atop another (Josephus describes this). In chapter 5 of John “there is” a sheep market with 5 porches. Not “there was.” Some people say this is vivid narrative, but that goes out the window because the normal tenses are resumed right afterwards.