For many years it was Taboo to mention on one company's programs that a featured wrestler appeared for another company, let alone won titles/accolades there. Many people think this was a Vince Jr thing but even before the national expansion it was uncommon, not unheard of, but uncommon, to mention that another wrestler had a career elsewhere. The most you would get is a recognition from the announcers that said wrestler was a big deal of sorts when they debuted, something along the lines of "I Cant believe SuperStar Billy Graham is here in the NWA" or something like that.
Now, in the 1980s when Vince Jr took over he basically built his entire main event and much of his upper card on established talents who were stars elsewhere. Savage was already appearing in his Macho Man character with Liz for his father's promotion and worked in the Mid South and Florida areas. Greg Valentine & Roddy Piper actually wrestled each other at Starrcade 83 in a Dog Collar match and soon debuted basically playing the same characters in WWE. Hulk Hogan was set to win the AWA Title from Nick Bockwinkle when a financial dispute with Verne Gagne made him available, he tweeked his already newly established hero persona that he established after years as a heel against Bockwinkle and Hulka-Mania was born. Ricky Steamboat was the same character for several years in the NWA before he signed with WWE. Some wrestlers came with minor tweeks, Rick Rude was basically the same character in World Class & NWA before joining WWE, as was Kerry Von Erich. Jake Roberts and Paul Orndorff went through little change from their NWA days before signing with Vince.
After awhile Vince started signing established talents but giving them significant character make overs such Ted DiBiase who was main eventing all over Mid South before joining WWE, or former AWA World Champion Curt Henning. This was an attempt to further the idea that no other wrestling company existed, or mattered, in the eyes of casual fans. Still, some stars were too big and popular in their gimmicks and too well known to fans to change. Vince never acknowledged any prior wrestling backgrounds for Tully Blanchard & Arn Anderson but he never changed their names or characters, likewise he kept everything in tact for the gimmick of The Road Warriors. Ric Flair was the first time Vince, Jr actually acknowledged a talent had wrestled elsewhere but even then NWA/WCW was never mentioned by name, Flair was referred to as an "international wrestling star who had won titles around the world and has now come to the WWF to win the biggest title of all, the World Wrestling Federation Title". It was also the only time before the heyday of The Monday Night Wars and the immediate aftermath of the purchase of WCW that a wrestler's prior career was mentioned on air as part of their promotion while working for WWE.
Likewise, as the other promotions slowly faded WCW remained, but largely ingnored past histories of its talent outside WCW. This was amended for Savage & Hogan, largely to play off their WWE feuds vs Ric Flair after he recruited them to sign with WCW. While WCW referenced past history from WWE in regards to these three with each other it pretty much ignored anyone's else's non WCW history pretty consistently, until at least the hey day of the Monday Night Wars (the best example being Kevin Nash year long undefeated streak as WWE Champ being used in promoting Starrcade 98 vs Goldbberg, as well as prior history between Brett Hart & Flair to sell the Souled Out 98 PPV).
With WCW out of the picture almost 15 years now and virtually all of the big names from that era basically gone (Hogan, Flair, Hart, HBK, Nash, Goldberg, Austin, Foley, Rock, all done - Taker & Sting almost done, etc) there really hasn't been much need to acknowledge performers past non WWE history on TV.
The reason is simple - the overwhelming majority, and I mean overwhelming, like virtually all, of the normal wrestling audience DOESNT WATCH any other wrestling. Even at its lowest point WCW was drawing 2 to 3 times the ratings TNA gets, and they get much higher numbers than anyone else, while only drawing a third to a fourth of what WWE is getting. Casual fans may have switched allegiance periodically between the companies but they watched, or at least were familiar with both and knew who the stars were and what their gimmicks were. Nowadays if WWE brings in someone from Ring Of Honor only about a tenth of their normal audience knows who that person was before joining WWE, so why bother being forced into using that same name & gimmick if WWE doesn't like it for that performer, virtually no one watching even knows what that performer used to be. Also, by WWE acknowledging the past career and achievements of wrestlers they sing from TNA or Ring Of Honor or other unknown indies they give major cred to those competitors, basically telling their audience that the talent in these companies is good enough we are signing them. You might as well ask your audience to watch those shows (if they can find them on TV). You don't build your brand and maintain market dominance by giving free publicity and credibility to your competitors.
In the case of the NWA and later WCW Vince & WWE couldn't avoid it. Too many people watched both shows to unilaterally just change wrestlers names and characters and never acknowledge who they were. Plus, the WCW audience was so big WWE potentially could reap a benefit numbers wise by signing top talents and using them in their established gimmicks with their histories on their shows, their fans from WCW might switch over and watch. What is to be gained by acknowledging the career of some guy from ROH, no one is watching them, and the few who do are wrestling die hards who probably watch you already, in very small numbers too boot.
This is why you don't see character pasts from other promotions being used anymore by WWE
