IIRC, 780BR was dropped because it fouled barrels badly and was quite difficult to clean out of the barrels. It was replaced by W-785 which, IMHO, wasn't much better. That too was replaced by WMR which was pulled before it had a chance to catch on. I liked it and found it useful in way too many cartridges for it to have had such an early demise. Well, I don't have to worry about running out of WMR for a while as I bought up every bit I could find.
FWIW, I can match or in some cases surpass factory specs with WMR in the .243 win., 6MM Rem.,.257 Robt., 25-06, .270 Win., and .300 Win. Mag. For some reason though, it doesn't work in the 30-06.
Looking through the various loading manuals, only Hornady and Nosler gave it any coverage. Without pertinent data, I guess nobody gave it much thought and like the dodo bird, it disappeared.
It does foul some, but nowhere nearly as bad as 780BR and W-785. With the two former powders, it took some fairly long sessions with JB Bore paste to get the gunk out.
Probably, about the time we get to liking the new powder, "Big W" will pull it because it didn't sell fast enough. We lost H-205 because Hodgden's didn't sell enough fast enough. Too bad as that was a great one in my 30-06.
That's the bad thing today about American industry, what's left of it. bring something new out and if it does not bring in instant millions, drop it before it has a chance to catch on.
Paul B.