There could be any number of reasons, here are a couple, less worry with corrosion without high levels of salt in the water, no need to add another thing to test for (salt), and then of course there is the issue of no expensive failure prone cell to replace, sure pumps have moving parts that need to be replaced from time to time, but they tend to be cheaper than salt cells, and often fail more gracefully giving some warnings before they outright die.
Ike