People must have figured out, it's not an easy racket and there's more money to be made elsewhere with less hassle. Dealing with inventory, clients, deliveries, PayPal and all, can be a real pain in the ass. Fees got higher. While buyer protection is strong, seller protection is literally non-existent even in clearer than the sun situations.
eBay changed a lot throughout the years, you can't sell a lot of stuff now, which you could in the past. PayPal holding your funds isn't too much of a help either, which wasn't the case back then. And i can imagine, if you want to bend the rules, it's a lot harder to game the system now, than it was back then. And when you need to play by the rules, it often means, it's harder to make (big) profit. And this forum was always about shortcuts, bending the rules.
eBay lost a lot of customers to Amazon for sure, but it's not just Amazon, it's Aliexpress too, people buy cheap sh*t directly from there nowadays. There are countless online shops, where you can get high priced items (e.g. electronics) often cheaper than on eBay, everyday eBay sellers can't compete with the discounts of big retailers. Or how many fashion retailers are there nowadays, who weren't there 5-10 years ago or were nowhere as popular as now? Competition is fierce.
And many changes happened in other industries too, which have an effect on eBay. For instance, take a look at gaming, most people don't buy physical games anymore, they buy games on Steam or similar. The situation is the same in two other industries: we mostly stream movies and music these days, less and less people buy CDs, DVDs or other physical discs. The way we consume media affected these industries, which affected eBay negatively.
Sure, there are many sections on eBay, but these are just a few big industries from the top of my head, where there were radical changes.
At this point i think, eBay is mostly good for selling/buying used stuff, in demand fashion items (sneaker reselling), collector's items, vintage or otherwise rare stuff or unwanted items, but this latter is never done on a big scale.