Pretty much the same reason why there are a ton of carrier exchange points in certain geographic areas of the SF Bay area. Or why the Westin Building in Seattle and One Wilshire in Los Angeles have become the defacto carrier exchange points/peering locations. Large numbers of ISPs began accretion in certain buildings in the mid to late 1990s and the process has continued through its own inexorable inertia.
IIRC when the Internet actually formed there were 7 places that you needed to be at in order to have settlement-free peering. Looking at various not-so-organized Wikipedia articles it appears that I am not recalling the earliest days. I think all of these still exist; MAE-WEST is still at 55 South Market in San Jose, for example.
At one point blekko moved into a brand new datacenter in Santa Clara which had only limited fiber connectivity. @ChuckMcM twisted their arm for a free interconnect to MAE-WEST so that we could move over our existing inexpensive transit and interconnect contracts. Everyone's in MAE-WEST.
I was just a pre/early teen when most of this was happening but I can remember reading about it and being amazed, after becoming interested after reading _The Cuckoo's Egg_.
Today, I'm a network engineer at an ISP. Go figure.
> MAE-WEST is still at 55 South Market in San Jose, for example.
Yep, and fiber runs from there to 611 Folsom and into Room 641A.
(Edit: It probably wasn't quite that early in the Internet's life but it was still very young.)
