[Jeritron]

They're also not going to hype him into something he hasn't been as of yet in WWE, just because he was for a stint in the small pond of TNA.
And that's not an attempt to bury TNA, it's an attempt to keep their product consistent. Christian was a huge deal in TNA before Angle and Foley showed up. That was because he was the top name in a very shallow pool of stardom.
WWE can't just honor him as a main eventer for his accomplishments there, when he was never that level in WWE.

By that standard, a returning Jeff Jarret would get a giant PPV return from WWE, or a first-time appearance of AJ Styles would be treated like the return of Jericho. WWE has to do things as WWE, and can't just cater to the very few people who watch both shows, by putting over TNA accomplishments. Even if it's in a roundabout way.
It's not like WCW incomers where Jericho and Big Show were treated as equals. That was a boom period, and other promotions shared equal amounts of followers for a time. It's a different environment today, and WWE reacts accordingly.