The problem with your question (and it's a good question) is that it can be answered on many levels... and without answering it on those many levels, you get an incomplete picture.

At the most basic level, the problem is one of parameter mismatch as described by Jaron Lanier in You Are Not A Gadget and Dan Ariely in Predictably Irrational.

From Lanier's perspective, "anonymity" is a spectrum, not a binary quality, and those with more of it hold those with less of it to different standards. Consumers on the internet tend to have complete anonymity while producers on the internet have conditional anonymity - you don't know who made that Oreos commercial, but you know they work for General Mills, for example. Self-promotion from a known quantity - General Mills, Snoop Dogg, Stephen Fry - is A-OK because everyone knows it's the identified pandering to the anonymous. The anonymous pandering to the anonymous is A-OK because flashes in the pan and "viral" posts are the bread and butter of social media. But because "viral" anything is the holy grail of advertising, the anonymous internet regards all polished things not flying the banner of their creation with suspicion and hostility.

From Ariely's perspective, we hold "friends" and "vendors" in different parts of our head and when our friends act like vendors, we feel betrayed. There are two different standards for them. This is why Amway and Avon are so insidious but so successful: your friends get a pass on behaviors that you would never tolerate from a business, but they're businesses marketed through your friends. In social media, "friends" are the other anonymous hordes like ourselves. "Vendors" are those who we buy shit from. The Internet is A-OK with vendors being vendors and friends being friends but the minute a friend becomes a vendor, fuck them in the neck. That's why people go to one Tupperware Party if any.

The next level is one of cynicism. The Internet was supposed to be the great equalizer, providing a forum for anyone with talent. Unfortunately, it provides a forum for anyone with a talent for promotion via the Internet. I wholeheartedly recommend Ryan Holiday's Trust Me, I'm Lying for a concise yet sweeping analysis of the "wagging the dog" phenomenon of Internet promotion. The mile-high view is that the process is largely dishonest and craven because dishonest and craven is the most effective and efficient way to do things. Consider: Elvis "made it big" but Rebecca Black "went viral." "Virality" (not virulence, because the ad industry needs its own terminology to distance itself from the fact that it's attempting to model itself on disease) is a much-studied specialty that is all about disingenuous promotion through dishonesty and even if "the internet" can't quote chapter and verse on the deceptions, they can smell a rat. Often when there isn't one there, in fact.

But really, at the base of it, the Internet hates self-promotion because the internet doesn't know who you are and doesn't care. You can shill your shit on Facebook. You can shill your shit on Tumblr. You can shill your shit anywhere that people expect shit to be shilled... but if you try and sneak into the crowd as a shit shiller, you will be pilloried.

'member back before Facebook had business pages? 'member how pissed people got if you promoted your business on your personal page? Notice how they don't do that anymore now that you can get a business page that plays by business rules?

That's pretty much the TL;DR right there - because self-promotion breaks "the rules" of sites like, say, Reddit.

If self-serve wasn't such a star-spangled clusterfuck everything would be A-OK. But they've re-jiggered it four times now and it's still a long way from useful.