Yosemite permits about 400 hikers per day to access Half Dome, Mt Whitney Main Trail is 100 day hikers, 60 overnighters. So I don't think there is as much "wiggle room" to create a daily lottery that would amount to much.
Additionally, most sea-level dwellers can climb Half Dome successfully without any acclimatization required. With Mt Whitney, on the other hand, it's prudent to spend several nights at 8k or 10k elevation to get better adjusted to the altitude. Half Dome's summit is only 8800', just a little higher than the Whitney trail head.
In a similar manner, the walk-in permit system for Whitney Permits gives people the chance that a daily lottery for Half Dome does. There are no walk-ins available for Half Dome -- no-shows are never made available to others. On Whitney, on the other hand, people serious about climbing who didn't get a lottery slot can usually get a permit, especially if they go mid-week. Such a person really needs to be staying at altitude in the area, so it is just a matter of driving to the Visitor Center and getting in line at the right time.
I'll put in my opinion here on the Half Dome permits: The primary reason for the permits is to avoid crowding on the cables. They could open it up to many more if they would give out permits for "Before 10 AM" and "After 3 PM" on the cables. With such permits, those people would be using the cables out of the peak times. Unfortunately, Yosemite managers aren't interested in trying anything a little imaginative.