Why can't the Old Republic be included in the new Star Wars continuity?

It doesn't interfere with or contradict the new Disney canon, so why did they put that one under the "Legends" banner?

Old Republic is my favorite area of the Star Wars universe in general, so it still makes me sad to see it as non-canonical. My hopes for Revan showing up in any of the new Star Wars stuff (preferably an Old Republic trilogy) have been crushed.

Because it ties with the Legends continuity, they can't make it canon without everything that comes after be canon.

Now the Old Republic as an era is mentioned, so it's there. But specifically what you're asking for, can't be canon. It can be brought back as can the characters, but will be different.

It is. Pre Vizla's ancestor got the Darksaber from a Jedi Temple during the fall of the old republic

They could potentially make it canon. They can cherry pick whatever they want from it, there's really nothing stopping them. But I imagine that's an undertaking no one is really considering atm. There's virtually nothing in any of the many pre-Republic periods. The new canon focus is almost exclusively in the Republic era, Imperial era and the immediate post-Imperial era. The franchise is largely contained within that span of roughly 40 years or so.

Because they are too stubborn.