Hussar
Legend
I'm not arguing that this is why new books shouldn't change canon...It was simply a counterpoint to @pemerton's assumption of the lore being readily available to anyone who preferred it.
EDIT: Now granted this is becoming less of a problem and once they have the Planescape stuff up for PoD I'll be one of the first to hand over my money to restock on all those books I loved (but didn't really care for properly) as a child... but the secondary market on ebay for Planescape was just to ridiculous for me to entertain the notion of buying back my entire collection.
But, now you're even further down the rabbit hole. Planescape material is setting specific and is not part of 5e core. Just a quick perusal of the 5e MM turns up that Salamanders are a slave race to Efreeti and hate Azers. AFAIK, this isn't part of Planescape. Is that part of Planescape lore that Efreeti tried to enslave Azers to build the City of Brass, failed and then enslaved salamanders to help them "unleash a war of destruction across the planes"? (MM 5e p265)? I'll admit my Planescape canon knowledge is extremely limited, but, it's the first I've heard of this. And 5e is replete with this sort of thing.
Which means if you want to use your Planescape material, you'll have to make changes to 5e lore. And, for some bizarre reason which I've never been able to understand, that's perfectly acceptable. But, using your Planescape material in 4e you'd have to make changes, but, that was unacceptable.

The older lore is right there for you to use. Either online in any number of sources, or, now, from the primary sources. You HAVE what you want. Why can't I have what I want too? Why does the lore you like have to be enshrined into the game, but, the lore you don't care for should be excised?