Vendor lineage
Proofpoint - the mail gateway, still
Email remained the way in long after the industry stopped finding it interesting.
Proofpoint built its business on email security at a time when the attention of the field was moving elsewhere. That timing turned out to be the point: email stayed the most common initial access vector through two decades of newer and more interesting attack surfaces.
The category is unfashionable in a specific way worth noting. A mail gateway is judged on what it silently prevented, and prevention produces no event anybody celebrates - while a false positive produces an angry executive whose invoice was quarantined. The incentive gradient runs against the control, and estates loosen mail filtering in response to complaints far more often than they tighten it after an incident.
That asymmetry, rather than any technical development, is why the same attack shape keeps working.