I’m pretty sure the speed asymmetry started for good technical reasons: through some signal magic, companies could deliver 48+ Kbps down and 33 Kbps up. ADSL took advantage of the technological landscape of the time (browsing and email were asymmetric) to deliver faster speeds where the customers cared, and indeed, my first DSL services were (if memory serves) in the 1.5/0.38 Mbps area, only a 4:1 asymmetry. My current service is ADSL at 9.3:1 (7.0/0.75), which is both notably closer to 10:1, and hasn’t qualified as broadband since the 4/1 Mbps definition went into effect.
Even though YouTube made video hit the web in a big way—they were there at the crossover point between better codecs and better bandwidth, plus some cleverness* on their part—most traffic was still downstream. The video being delivered was much larger than the return traffic that acknowledged receipt of the video.
The restricted upload rates are thus firmly grounded in historical reality, and they persist today because, I suspect, of two reasons.
One, there’s obviously a chicken-and-egg problem where uploads are less frequent because upload rates are low, and the rates are lower because uploads are less frequent. There’s a natural tendency for uploads to be less frequent anyway (how many funny cat pictures do you look at per picture you upload?) but low upload rates discourage actual upload usage in and of themselves.
Two, I think ISPs are rewarded if they keep upload rates low. Settlement-free peering has traditionally required each side to send “about equal” traffic as the other. If an ISP strongly encourages downloads through 10:1 or more asymmetry, then they will never come close to sending “about equal” traffic out of their network... and they can demand payment from anyone who wants access to their customers, such as Netflix.
I still believe that ISPs should be charging their own customers enough to support their own customers’ data requests including adequate network investment, but that doesn’t invalidate the reality.
As for 10:1 specifically, I can only speculate. It may be, that’s simply the size where sending email and uploading to Facebook “doesn’t seem to take too long” for users. And if more people sent more video to Facebook, then ISPs may reshuffle their plans to provide more upstream “so you can Facebook.” Regardless, in the absence of an obvious technical reason, I must assume it serves a specific marketing purpose.
* At one time, they showed 320-pixel video in a 425-pixel player. Although scaling technically hurts quality, it crossed the gap between “small” and “nicely sized,” looking much better on 1000-pixel browsers.