Because the Seavee did not perform well with a single POD. So they stopped making it. It didn't offer any performance or fuel economy advantages over the outboards.
Even in the Seavee 390 which is offered inboard, IPS and outboard, there's little reason pushing buyers to the IPS. The typical high performance center console buyer is only mildly excited by it's 1.9 mpg and doesn't really feel the need for the handling. The IPS with 870 total horsepower has a top speed of 50 mph, triple outboards at 900-1050 total max out around 70 mph. Or go to quads up to 1400 hp. Oh and with twins or triples you're not that far under the economy of the IPS. IPS brags about how the 435's (Total 870 hp) are the equivalent of 1200. That's 1200 hp perhaps in an inboard but they aren't close to 1200 hp of outboards. The purchaser is experienced with outboards. When the outboards die or get damaged very easy to just drop another on. For fishermen that is huge as the last thing they want to do is lose fishing time. Now the IPS does have advantages over inboards in performance and fuel consumption but that's still a very small part of the business.
The other thing with the IPS in a situation like the Seavee, is that you lose a lot of the interior of the boat......Space where a head is usually located, storage, etc.....
That's a huge one especially in a boat like that which is a "Luxury Edition." The outboard has a very nice head.