Infrastructure. A slate brings an infrastructure with it - for campaigning, for communicating your ideas with the people you'd like to connect with, for getting signs and websites done and holding events. It's a machine - and it's one that an independent candidate has a hard time creating on their own. Stephen Andrew didn't run as a slate this campaign - but the infrastructure was slate like. It was doable because this byelection wasn't about attempting to fill 9 positions (8 councillors plus mayor). It was also doable because this was SA's third kick at the can, so there's been a lot of learning the last two times. In a general election maybe 3 or 4 of the candidates (absent slates) can muster that kind of campaign - because it takes a lot of man power.
Unfortunately, slates also mean poor candidates are uplifted and are more likely to be elected because of the power of the organization behind them. I think we've seen that around the table - worse yet, there's an argument to be had that some who ran independently, weren't actually independent but were tangential to the slate. Fixing the problem is tricky. A ward system might be helpful - an independent with strong community roots might do be better in a ward system where "city wide" recognition is no longer needed. As an example, Marg Gardiner might have done much better last election if James Bay was a ward as she has deep community roots there, but is less well known in other parts of town like Fernwood. A ward system might also prevent certain neighbourhoods from being over represented on council. Right now, there's an argument to be had that Fernwood/North Park are over represented on council. Rules around how signs must look - or even the maximum quantity of signs allowed, standard web formats, and universal minimum media coverage might help level the playing field a bit.