Summer Glau has been on so many cancelled shows because she gets hired on so many shows.
She only gets hired on so many shows because her shows keep getting cancelled, freeing her up to be hired on other shows.
And there are many, many actors you could say the exact same thing about. Most shows get cancelled. That is the norm, not the exception. There are far, far more short-lived TV series than long-lived ones. So any actor who's working steadily and prominent enough to be cast in lead or recurring roles is likely to go through a number of short-lived series.
The only reason Summer Glau gets noticed for this is because she gets noticed, period -- because she's a striking and memorable performer and people pay more attention to her career than they pay to a lot of other actors who've had very similar career arcs.