Because most internet only Verizon devices sit behind a NAT firewall by default. While your phone may be fortunate enough to have a pubic IP, Jetpacks, USB modems and 4G LTE Routers are not. You need to use the well known work around to NAT firewall configurations to get your Xbox game hosting services to work correctly.
Work arounds include:
1. Setup and configure a VPN connection to a VPN server with Public IP addresses
2. Purchase a static IP from Verizon for $500
3. Migrate from a Jetpack to LTE Internet Installed which comes with a free public IP
4. Stick to the hotspot service on your phone
5. Terminate with Verizon and begin service with a provider that offers public IPs for free/cheap