Why are there no large turboprop passenger airliners?
Large airliners don't fly at slower speeds.
It's simple economics.    

If you have to connect two destinations 10,000 miles apart, you'll do maybe 2 round trips a week. Your passengers have no other choice, except a flight that takes even longer. You also have the luxury of flying between two very large cities.

If you have to connect two destinations 100 miles apart, you can do 3+ round trips a day. Your passengers choose and pick among a dozen other carriers that do the same, and many will just go by car or bus. And at this distance, at least one of the cities being services is small.

As a result, a single 50-passenger turboprop running a local route can transport as many passengers as a 500-passenger ultra-long-range turbofan. At the same time, the local route will have far fewer passengers per day.

Fast long-range turboprops are possible (see the Tu-95 and -114), and only a little slower than turbofans. But they're still louder. All in all, for large planes on long trips, both the flying public and people on the ground choose quieter faster planes over somewhat more fuel-efficient ones.

Extra noise for an hour is less of a pain, speed matters much less, and the smaller turboprops working these routes are less of a pain on the ground too. The fuel economy gap isn't all that large.