There are not that many people capable of a secure design & implementation. But that is just the tip of the iceberg; any network promising anonymity requires a large number of nodes (and high traffic), and that number must stay significantly larger than the number of malicious nodes that try to deanonymize the network.
Adoption remains a huge challenge (and not really one you solve with technology or skill). It doesn't help that design decisions that improve security can degrade the user experience, leading to lower adoption and lower security. Sigh.
I left it as "network effect". Probably too opaque.
It's like there was a window in the early 00s, when stuff like Tor, Freenet and I2P could recruit enough nodes to be viable. Remailer networks were just too damn hard to use. I mean, they were based on PGP plus mix networks.
Adoption remains a huge challenge (and not really one you solve with technology or skill). It doesn't help that design decisions that improve security can degrade the user experience, leading to lower adoption and lower security. Sigh.