Back in 2007-2008, Dodge was seriously considering a convertible. As evidence of this, note the location of your seat/lap belt harness anchoring points. However, because of cost, it never happened. If you want a convertible, you'll have to have an existing Challenger converted by a specialty shop. Now, FCA is content just to ride out the current model, with little or no changes, until it no longer makes a profit. FCA's focus is on money-making Jeeps and Ram pick-up trucks.
The company where I had my conversion done is in legal talks with FCA to produce a limited amount of the convertible conversions to be sold as new vehicles at select dealerships. Sounded like they may do some of them over the next 2 years. The place I was at would really have to tool up to make any numbers. They told me they have made about 500 total since 2009. I figured it was a lot of experience, so I trusted them with mine. They had 2 others there that had less than 20 miles on them from dealers. A nice red SRT 392 they were doing a black top on, and a blue one that was still a hardtop when I left. I know a dealer in Virginia sent them the red one. Someone told me they saw a grey one with red interior and red top on Facebook at a dealer.
Drop Top Customs out of Florida has done these from day one when the challenger came out, now they do Chargers as well as 300's. and other models too.