Why are IT operations, OT operations and network administration named as stakeholders in their own right?

Question

Accepted Answer

Because they are where the architecture becomes practice — they own the security tools and the running of them — and network administration in particular, since architecture work usually means deep intervention in network topologies and configurations.
IT operations is responsible for the IT security tools and building blocks, and for organising their operation. That is where the largest part of a cyber security architecture is actually realised. Which sets a design criterion that paper architectures habitually ignore: a good architecture mitigates risk and takes cost, operability and supportability into account. A design that cannot be run with the staff available is not a secure design — it is an insecure one with good intentions, because unmaintained controls decay.
OT operations does a job similar in kind but different in constraints — availability and safety first, maintenance windows measured in years. Because of those peculiarities the relationship has to be a close collaboration rather than an instruction; security requirements imposed on a production plant from outside tend to be either refused or quietly disabled.
Network administration gets a separate mention for a concrete reason: cyber security architecture generally brings strong interventions into network topologies and configurations — segmentation, zones, new choke points, routing and firewall changes, VPN and proxy paths. Those are their systems, their change windows and their outage risk. Involve them at design time and you get feasibility feedback while it is still cheap; involve them at implementation time and you get a queue.
More broadly: depending on the security paradigm, other administration areas (OPS) take on a key role too — identity administration under an IAM-centric model, endpoint management under a zero-trust one.
Go deeper:

NIST SP 800-82 Rev. 3 — Guide to Operational Technology Security — the reference for securing OT without breaking the process it controls.
 Thomas Roth — SCADA: Gateway to (s)hell (34C3) — a live look at the industrial gateways that connect OT to IP networks, and how little they resist.