How come most fan sites for MLP hardly talk about anything before G4? I have seen fan sites other shows where earlier generations are talked about despite those not being around at the time to see them, but not this one for some reason. Were they just not that exciting? Maybe it was because there was no internet message boards at the time. Others might say that they were like the WNBA where most wouldn't even know about it until a fan mentions it.

The explosion of the modern internet fandom was tied very specifically to the G4 show, so one part of it was that most fans weren't directly involved with, or particularly aware of, the older material. Consequently, it mostly defined itself as the fandom of Friendship is Magic, not of My Little Pony generally. (In contrast, consider the Transformers fandom, which has existed for most of the franchise's history and thus tends to define itself in terms of the overall franchise rather than any one specific iteration.)

The other part of it was most likely the franchise's existing reputation as little kids' fare. One way which the fandom has dealt with this internally, which is very clear if you look at how older gens are usually discussed, is by offloading it to the series' past iterations. Essentially, the line goes that, sure, the older MLP stuff is kiddy and shallow and bad, but we don't like that, we like the current gen, which is clearly better and more interesting.

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Because Hasbro isn't selling it.
And therefore, Ads aren't TELLING THEM to love it.

There's been a MLP community almost as long as the internet's been around. However, this has always been catered more towards collectors and is primarily toy-focused. There was/is (of course) discussion on the movies/specials/shows, etc., but it always took a backseat to the toys. Fans of the older generations initially embraced the brony fandom, but that went south pretty quickly. That's why there's not much overlap despite all of us being in the same fandom. Some sites I recommend:

https://mlparena.com/index.php (message board; registration is required to view)

https://www.mlptp.net/index.php (message board)

https://www.strawberryreef.com/ (collector site; note the links section for even more MLP websites)

https://www.toysisters.com/toy-guides/my-little-pony/ (collector site)

As for why most brony sites don't mention the earlier generations, or do so derogatorily, it's mostly ignorance and misguided masculinity. G3, the immediate predecessor (and especially G3.5) was definitely focused towards very young girls, and that's the perception most people had of MLP prior to 2010. The early brony fandom wanted to distance itself from that image as far as possible.

Lauren Faust's inspiration for G4 was G1. That seems to get glossed over... Most bronies aren't old enough to remember the 1980s. G1, having the benefit of being made back then, is darker and more badass (and cheesier) than FIM could ever have hoped to be. Some examples:

https://derpibooru.org/images/2517066

https://derpibooru.org/images/421289

https://derpibooru.org/images/446450

It doesn't help that Hasbro themselves have made fun of the earlier generations. This was originally posted on Hasbro's official channel: https://derpibooru.org/images/2089000

Note that 'Sprinkle Bottom' is actually G1 Cupcake.

I will admit that everything between G1 and G4 wasn't that good, which is why some ended after one season. Saying that they were long commercials was very common for kids shows as that's still pretty much the case today. For those who don't know, MLP Tales was actually G1.5, not G2. There really was a G2, but it was never made into a show unlike the others. Still, I will always find a special place for G1 for having so many dark moments that even generations after it would never show. Another thing that made it special was that it had the one thing that no other generation will probably ever try to duplicate and that is human companionship. My only guess to why they had it was because it was still something new at the time so having them was made most likely to get everyone used to it. I really do wish that this idea was brought back in more recent generations such as G4, but knowing who was in charge, it would never happen. This is why I will always look at it as a missed opportunity. One other thing, I think bring back human companions such as rebooted versions of Megan and possibly her siblings Danny and Molly would have been a better fit and have the ponies stay ponies rather than having a spinoff such as Equestria Girls where they are humanized, though I won't say that it was totally bad. Then again, I'm probably one of the few who knew how to picture what it could have been if it had happened. The least that could have been done was give Megan a real appearance in the crossover comic series, but even that was too good to be true. Nonetheless, that's my latest fanfics "Unexpected Arrival", "The Inconvenient Truth", and "Danny and Molly Join In" does give the idea of what certain episodes would have been like if that was to happen.

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Some even used the scene from the episode "The Magic Coins" where Megan is shown in the fire be it with or without Wind Whistler by her to create a caption. For me, I use it to show how little love she has to this day and is almost forgotten to an extent. Also, I feel that if the fire does incinerate her, she most likely won't be missed from it. Also, there is a good chance that her death would be cheered if that were to happen. In a way, it's sort of like saying that she was tossed down the memory hole after G1 ended after serving her purpose of only being there to get everyone used to the franchise when it was still something new.

G1 will always be known for this.