I think rocks are a big part of it. I seem to recall (don't have a cite handy) an estimate of something like a 1 in 3 chance that Viking 1 would have hit a disabling rock at its site (it had less than a foot of ground clearance underneath). At the Pathfinder site, which was even more rocky, Viking would have had a hell of a time.

Probably more sensitive than airbags to minor terrain roughness, like if the initial landing were on the lip of a small crater like "sleepy hollow" -- airbags would roll away, Viking (or MPL) would have been at a probably fatal tilt.

On the other hand, given how flat and rock-free Gusev is, I bet a "dumb" retrorocket system like Viking or MPL would have been fine.

Developing a "smart" retrorocket landing system is a big priority for the 2009 super-rover, I believe.