Kubernetes: After port-forwarding, I can access a service via "localhost" but not via "hostname"

For the main.go code at the end of this question, I ran the following commands to run it on a kubernetes install (on a PC):

1. docker image build -t myID/go-demo:1.2 .

2. docker image push myID/go-demo:1.2 # Pushed up to DockerHub

3. kubectl run demo2 --image=myID/go-demo:1.2 --port=19999 --labels app=demo2

4. kubectl port-forward deploy/demo2 19999:8888

   Forwarding from 127.0.0.1:19999 -> 8888 Forwarding from [::1]:19999 -> 8888

Then, in another tmux(1) terminal, I confirmed the service was LISTENing:

user@vps10$ sudo netstat -ntlp | egrep "Local|19999"
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp        0      0 127.0.0.1:19999         0.0.0.0:*               LISTEN      736786/kubectl      
tcp6       0      0 ::1:19999               :::*                    LISTEN      736786/kubectl

But here's my problem, noticing success with localhost and failure with hostname -- vps10:

user@vps10$ curl localhost:19999 # Works.
Hello, 世界

user@vps10$ curl vps10:19999     # Fails.
curl: (7) Failed to connect to vps10 port 19999: Connection refused

From the above, the issue appears to be that the service is listening only via the loopback interface, and if that's indeed the issue, what do I do to get it to listen on all interfaces (or on a specific interface that I specify). I'm not a kubernetes or go expert (this example is from a book actually =:)), so please supply commands if necessary. Thank you in advance!

HTTP server code:

package main

import (
        "fmt"
        "log"
        "net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintln(w, "Hello, 世界")
}

func main() {
        http.HandleFunc("/", handler)
        log.Fatal(http.ListenAndServe("0.0.0.0:8888", nil))
}