It's simple really - in terms of technical expertise in online content delivery Sky are novices - and seriously lacking in comparison to their competitors.
The issue they have is that they're attempting to build this in house (I know more than one person working over there) and because this is new to them, they're making continuous errors which they're not able to instantly fix.
The solution, in my opinion, would be for them to cease developing Now TV in house, and partner up with an expert in delivering content online. Recently WWE Network launched in partnership with MLBAM - there were issues in launch week, but since then problems were quickly ironed out, HD on demand works now flawlessly and high volume 720P live content is now also being delivered without any issues (after initial launch hiccups).
Bottom line - for these errors to continually happen on Now TV and Sky Go with no fix in the offing, whilst still charging people for a service that can't handle the load is unacceptable. In its current form it is not fit for purpose.