Any reason why the Defiant has "NX" in its registry rather than "NCC"?
Is/was this ever explained?
The Defiant was originally an experimental design that Sisco was the project lead on. It was a purpose built Anti Borg ship. Basically to paraphrase Chuck over at SFDebris "It's a set of guns strapped to an engine with a chair in the middle". The Defiant was originally the first and only ship of its kind built and was mothballed before it completed its testing trials. Sisco dusted it off to deal with the Dominion. Because it was commissioned as an experimental vessel it got the NX designation, similar to the US Air Forces "X" plane designation. Somewhere around seasons 3 and 4 Starfleet does begin full scale production of the Defiant design, so we eventually do see several of the ships with NCC designations. There are 2 named and maybe 2 un named that are seen in DS9 and about 4 or 5 un named that appear on screen in Voyager. (2 appear in Eye of the Needle).
Technically the Defiant really should have switched to an NCC registry later in the series as the other ships entered service. Much like the Excelsior did. But some truths of production economics left it as NX. Most of the shots you see of the Defiant are stock shots generated before regular filming on each season begins. They build up a library of such shots over the seasons (hence one of the reasons why the effects get more elaborate over the seasons. They have a deeper library of established shots to draw from and piece together precisely what they want.) But if they change the registry of the ship, they have to throw out and reshoot all of those stock FX shots. TNG ran into a little bit of this when they replaced the 6'1701-D model with the newer easier to work with 4' one. This is also ultimately why Voyager never changes. Why it always resets to status quo in appearance. To do anything else throws out too much expensive effects work. It's the downside to practical model work. Contrast that with the Ron Moore run Battlestar Galactica where the master CGI model of the ship was able to be evolved as the show went on. By the time you reached the last episode every single weapon hit from the pilot onwards was reflected in its skin. They could reskin their animation framework with new graphics.
And you will enjoy the Season 5 finale. Short of the series finale "What We Leave Behind" it's probably the best season ender.