That's NOT true at all. There are Gnolls with Assimar blood in them out there AND in 4th ed they talked about playing non evil Gnolls. They don't have to be evil to exist. I mean haven't you heard of Tieflings? They simply have demonic blood in them but it doesn't mean they can never ever be neutral or even good.
Agree to disagree? Cause I'm not agreeing they can't be good in the Forgotten Realms, where all kinds of whacky crap happens anyway and where ONE good Drow has given birth to a Good drow godess and her followers...Cause I don't remember jack about Ellistrayee before Drizzt came around...So I say nay to you. MEATBALL THE GNOLL SHALL LEAD THE WAY TO GOOD, ERR NEUTRAL GNOLLS WHO LIKE TO EAT LOTS OF STUFF!
Well, there were different takes on what Gnolls were in 3rd edition. What it said in 4th edition about Gnolls would apply to Nentervale or whatever the setting is called ("Points of Light") where I am given to understand they are neutral. Sure, because the new original setting was "core", 4e tried to force all other D&D settings to adhere to its new lore. Within THAT setting, by all means Gnolls can be good aligned.
The lore of what a race is does not have to be consistent from world to world in D&D. In fact, it shouldn't be consistent world to world-- what exactly is the point of having different settings if all the peoples in it are completely identical across them.
Now-- certainly in previous editions of Forgotten Realms, possibly due to the "spell plague", there were neutral or good Gnolls in the Forgotten Realms. However, the effects of that were reversed. Currently within Forgotten Realms, and we can only speak for Forgotten Realms, Gnolls are very much akin to the Rakasha.
Given the fact that someone working late 3rd edition and in 4th edition had a whole thing for Gnolls well above and beyond other even more popular monster races, I don't doubt there was a big attempt to push them as a not unexpected "good" race. Afterall, they were printed as a PC race in both Races of the Wild and within 4th Edition. However, that was a massive reversal from their understood origin and nature before that point.
Just comparing Gnolls vs. Orcs
We went from "Gnolls and Orcs are always evil without exception, do not stay your blade even in the case of babies, any idea of playing one is obviously a joke and we will treat it as such" in 1st edition, to "Gnolls and Orcs can be heroes if you really insist, but everyone will hate them and try to kill them and here is some really imbalanced stat blocks for them" in 2nd, to "Gnolls are totally evil, but Orcs can be noble and good at time even if they aren't half-human.. oh, wait, sorry just kidding about Gnolls, turns out they can be good guys too" in 3rd to "Gnolls are sometimes good and a very common hero race while Orcs are basically evil to the last one and there is no point but to make the most token effort towards allowing someone to play one in an unofficial nonsanctioned game" in 4th to "Orcs can sometimes be good and so we will print a totally badly designed racial template for PCs to use if they really insist, but Gnolls are entirely evil down to the last one so we will provide no template." in 5th.
But given the current concept for them, if it is impossible to have a good or neutral aligned Rakasha, it is equally impossible to have a good or neutral aligned Gnoll. Within the current incarnation of Forgotten Realms. And for the exact same reason-- both are infused with chaotic evil energies.
If you are in other D&D settings, the rules are different because what Gnolls are within those settings is fundamentally different. And thus, if one asks if it sucks they refused to make a stat block for Gnolls in Volo's Guide?... Eh, kind of. The stat blocks in the book are so crappily made, poorly balanced and generally crap that I don't recommend anyone use them. Someone who actually cared about the subject could have done a better job in a week than the so-called "professional game designers" did in the 3 months they worked on the damn thing.
Hobgoblins that are literally the crappiest possible race to choose for any class (particularly the ones they are stated most often being) except Wizard vs. Bugbear that gets increased damage on all weapons, free sneak attack and powerful stat boosts.
Be glad they didn't print an official stat block for Gnolls because then you would be stuck with an official one that you are required to use if you want to play the race that is so absolute garbage and sets you back an entire level compared to the rest of the party so-- naturally you wouldn't want to use it.