Money. Bandwith, storage space, server maintenance, and all that other shit costs money. The world runs on money so if you have a popular site you're very directly encouraged to monetize it because every second you don't is a second you're losing money and/or a second that someone else spends figuring out how to do your idea profitably.
The centralization is a result of how corporations work. Money flows up in a market system, so those who are most efficient at exploiting the market and their workers and have a highly desirable product will eventually amass more money than those who don't, allowing them to make deals and secure assets that allow them to more easily exert control on a given market.
The censorship of sexual material is simply a result of a few hundreds of years of abrahamic values becoming the dominant set of morals pretty much globablly, and as such traditional advertisers and companies find the material undesirable and upsetting, and consumers are either unwilling to support such materials or will do so until they are found out and shunned by their peers.
Assuming humanity survives the next 10 decades I imagine the puritanical beliefs will subside, so you'll see a boom in porn in your lifetime, but it's unclear as to if we'll ever create a society in which money doesn't drive the world and doesn't slowly >>34821250
We're at a time of great financial instability so a couple big things are happening around us that would have been set apart by months or years in more stable times. It's not that pastebin saw pornhub nuke their shit and wen't "aw hey that looks cool we should do that", but more likely that they took a look at their numbers and decided that now is the time to make big plays and set themselves up to do well in the future by playing it safe.