The biggest limiting factors for me are:
Time-If there are a number of places I need to go in a day, there simply may not be time to do it on a bike and take care of whtever else I have going on.
Cargo Capacity-As a private contractor, there are days when I need to carry large and/or a heavy load of tools. Normally I can make one trip in the car carrying the tools I will need for a project, and bike commute the rest of the time, leaving the tools on site. Then pick up the tools at the project's completion.
Weather-To some extent. I won't ride if it's lighting or if the wind is blowing over 30, particularly if it's gusting, but under these conditions I'm not likely to be going to work anyway.