They are more expensive, slower, become obsolete faster, have worse keyboards, worse monitors, etc. Trial lawyers need high-end systems for many of their needs, with CD ROMS, fast performance, good graphics, etc. In laptop computers, this combination is often particularly expensive. If most of your work is home and office, then a desktop machine each place is better. For actual courtroom use, while everyone thinks a laptop is best, a desktop is better, and most lawyers will set up a desktop computer at a trial.
Laptops are great for motions, however, and for mediations.
Also for depositions.