The exact date of Alexandria’s destruction ranges from 380 to 391.  The majority of sources cited Theodosius I as being responsible for the order.

In one source I read, Theodosius destroyed all of the City of Alexandria saying, “Let their blood reach my knees.” 

This is from Wikipedia: 

The Serapeum of Alexandria was closed in July of 325 AD, likely on the orders of the Christian emperor Constantine.^{[citation needed]} Emperor Theodosius I (379–395) gradually made pagan feasts into workdays, banned public sacrifices, and closed pagan temples. The decree promulgated in 391 declared that “no one is to go to the sanctuaries, [or] walk through the temples”, which resulted in the abandonment of many temples throughout the Empire. This set the stage for riots in Alexandria in 391 (although the date is debated). According to Wace,^[1]

The Serapeum was the last stronghold of the pagans who fortified themselves in the temple and its enclosure. The sanctuary was stormed by the Christians. The pagans were driven out, the temple was sacked, and its contents were destroyed.

The Serapeum was destroyed by Roman soldiers in 391^[3][4] and not rebuilt. After the destruction, the Monastery of Metanoia was established,^[5] and a church was built for St. John the Baptist, known as Angelium or Evangelium. However, the church fell to ruins around 600 AD, restored by Pope Isaac of Alexandria (681–684 AD), and finally destroyed in the 10th century. In the 20th century, a Muslim cemetery, Bāb Sidra, was located at the site.^[1]–Wikipedia

The article continues: 

The destruction of the Serapeum was but the most spectacular of such conflicts, according to Peter Brown.^[6] Several other ancient, and modern authors, instead, have interpreted the destruction of the Serapeum in Alexandria as representative of the triumph of Christianity and an example of the attitude of the Christians towards pagans. However, Brown frames it against a long-term backdrop of frequent mob violence in the city, where the Greek and Jewish quarters had fought during four hundred years, since the 1st century BC.^[7] Also, Eusebius mentions street-fighting in Alexandria between Christians and non-Christians, occurring as early as 249. There is evidence that non-Christians had taken part in citywide struggles both for and against Athanasius of Alexandria in 341 and 356. Similar accounts are found in the writings of Socrates of Constantinople. R. McMullan further reports that, in 363 (almost 30 years earlier), George of Cappadocia was killed for his repeated acts of pointed outrage, insult, and pillage of the most sacred treasures of the city.^[8]

And based on what I read in A World Lit Only By Fire, by William Manchester, there is much more to the picture that gets ignored. 

1 – 100 years later, the Dark Ages had begun. 

2 – 30 years after that, St. Patrick was sent to Ireland to destroy their Educational System, which mirrored Alexandria’s. So much was going on in Ireland at that time.