This can be a tough sell, especially in a society where lawyers are held to a different standard from other professionals. There are few home contractors who will give away every tenth townhouse for free to a needy family that couldn't otherwise afford it; these professionals operate on a tight profit margin, and simply can't afford to be profligate with their money and time.
The same rule applies to lawyers who are struggling to establish their practice, and need to make every hour count toward the bottom line. If taking on a pro bono client means not being able to make your mortgage payment, there's no ethical rule on earth that says you can't decide otherwise.
If, however, you're hearing this criticism on a regular basis, there may be something to it. Most likely, your fellow lawyers know that you have attained a certain amount of success with your practice, and that you can afford to “give back” to the community rather than saving up for a third sports car or a second vacation home.
Once again, there's no law that says you have to take on pro bono work (although your state bar association may pressure you on this point), but you have already profited enough from your practice that you can afford to do so. If it creates good will among your colleagues (and potential adversaries), it may well be the case that taking on more pro bono work is a good idea—and it may even make you feel good inside!
And, of course, getting a reputation for sharing your legal knowledge with clients who could not otherwise afford your services can be a clever, but subtle, way to promote your law firm.