Twin keels are less efficient. Same reason there are not many biplanes around now. One wing is better than two.
The overall weight of the keels and therefore the overall displacement
of the boat is higher, or the center of gravity is higher. Sailing performance is impacted either way.
If you run aground, you cannot reduce draught by heeling the boat.
Applying bottom paint
is more of a P.I.T.A.
There is much more torsional strain on the hull
to keel joint.
not everyone is so worried about sailing efficiency,
Draught is lower, so less likely to run aground
You can park on the beach if the tidal range is sufficient
You can do simple jobs on the hull without craning out
You don't need a cradle
if you are craned out.
You can use a drying mooring
The choice depends on where you sail and how you sail.