There are many factors that go into choosing dosing intervals. Some drugs are described as "bactericidal" vs "bacteriostatic". Some bactericidal drugs have a "concentration-dependent" response rather than a "time-dependent" response, which means that they are more effective when given a large dose for a short duration rather than a smaller dose for a longer time. Some drugs also continue to inhibit bacterial growth even when it is no longer detectable in blood (postantibiotic effect)

There is a class of drugs called aminoglycosides (tobramycin, streptomycin, amikacin) that are often taken at long intervals. They have an initial concentration-dependent bactericidal effect when first taken, then continue to provide bacteriostatic effect for many hours. Infrequent dosing also helps reduce toxic side-effects (hearing damage).

Also drug makers try to make drugs which can be taken as infrequently as possible while still being effective in order to improve patient compliance. They want you to take the drug as directed and not create a resistant super-bug. If the drug is effective when only taken once or twice per day, particularly in oral medication for an outpatient, this is preferable to a drug that is taken multiple times per day. Its just human nature to forget a dose here or there and to be less motivated to remember doses on time once you start feeling better.