- VPN protocols need to provide encapsulation that TLS doesn't provide
- Of course, it's possible to tunnel VPN over TLS to get the same effect!
The name Virtual Private Network suggests the answer to your question. A true VPN protocol emulates a Network, which implies the ability to route multiple traffic types and ports simultaneously over a single channel. That's why with a VPN protocol like IPSec you can end up with, for example, TCP packets encapsulated within ESP packets encapsulated within other TCP packets. (See also What's the difference between SSH and IPsec?)
By default TLS doesn't provide this capability. TLS takes bytes, encrypts them, and transfers them to the recipient who can decrypt it. That's it; everything else that's to be done has to be done by the application. Under HTTPS, a web server and client exchange HTTP data bytes over a TLS channel. Under IMAPS, a mail server and client exchange IMAP data bytes over a TLS channel.
In order to have generic "Network" connectivity over TLS, you need a client and server able to do that encapsulation and decapsulation on either end. You don't get it for free.
That's not to say you can't have a VPN that runs over TLS. There are many products that do it; even mainline vendors like Checkpoint support both IPSec and TLS-based VPNs. In some cases the TLS is simply encapsulating IPSec datagrams, so the actual "N" is via IPSec but the TLS gets it across the Internet.
Note that in some cases a sophisticated adversary can infer when a TLS connection is being used for VPN based on the pattern of communication. There's more to Operational Security than encrypting bits!