Why is the network configuration I set in /etc/network/interfaces ignored on Ubuntu 17.10?

Networking is handled by netplan by default in Ubuntu Server 17.10 and later. I suggest that you edit the /etc/netplan/01-netcfg.yaml file to read:

# This file describes the network interfaces available on your system
# For more information, see netplan(5).
network:
  version: 2
  renderer: networkd
  wifis:
    wlp2s0b1:
      dhcp4: no
      dhcp6: no
      addresses: [192.168.0.21/24]
      gateway4: 192.168.0.1
      nameservers:
        addresses: [8.8.8.8,192.168.0.1]
      access-points: 
        "******":
          password: "**********"

Exit and save your changes by running the command:

sudo netplan generate
sudo netplan apply

Please note and follow the spacing and indentation. Also note that the SSID and password are in between quotes ".

Comment out all the wlp2s0b1 stanzas in /etc/network/interfaces and reboot.

Any improvement?

NOTE: The exact method to set a static IP address for a server with netplan and wifi is hard to find. We may need to tweak the settings a bit.

I suggest you to completely remove netplan package and use networkd by creating config(s) in /etc/systemd/network/ directory. Netplan exists only in Ubuntu and there are no one in any other distros.