I think the main issues are (not in any particular order):

safety
time
level of effort / concerns about being fit or healthy enough
needing a car at work
logistics (have to get kids to/from daycare, etc)
concerns about weather (heat, cold, rain, snow, storms)
wanting to be "fresh" for work
safe place to keep bike at work
Don't have a bike
They like driving
concerns about image


There are probably others. Though the idea of having to wear spandex being a deterrent to potential commuters has been brought up multiple times, I personally don't buy it. About 1/3 of American adults own or have easy access to a bike. Even if they don't have a bike now, the majority rode them as kids. Most of these adults and kids ride spandex free.

If anyone were to tell you they don't commute by bike because they don't want to wear spandex my guess is that if you dug a little deeper, the real reason(s) are in the above list or put more simply, they have no interest in it. The spandex is just a convenient reason they can give that won't reflect poorly on them in any way.

I also believe that of those people that own bikes that are in reasonable condition and that fit them, their bikes are more than likely appropriate for commutes up to 10 miles or more. Some minor changes might be required.

I think riding recreationally (that would include getting ice cream or dinner) and commuting are complimentary activities. By promoting one you are promoting the other.

Running errands where you have to carry stuff, especially a fair amount of stuff like when grocery shopping I think takes more of a commitment.