First reason was that it was an array of connections you loop through to broadcast some data. We had around 2000 active connections and needed a less than 1000ms latency, with WebSocket, even though we faced connections drops, client received data on time. But in SSE, it took many seconds to reach some clients, since the data was time critical, WebSocket seemed much easier to scale for our purposes. Another issue was that SSE is like an idea you get done with HTTP APIs, so it doesn't have much support around it like WS. Things like rooms, clientIds etc needed to be managed manually, which was also a quite big task by itself. And a few other minor reasons too combined made us switch back to WS.
I think SSE will suit much better for connections where bulk broadcast is less, like in shared docs editing, showing stuff like "1234 users is watching this product" etc. And keep in mind that all this is coming from a mediocre full stack developer with 3 YOE only, so take it with a grain of salt.