My guess would be price...
...Niks philosophy is best quality for fair price. Reverb would definitely increase the price quite a bit.
This sounds plausible.
Reverb significantly complicates the design...extra transformer, more tubes, revoicing the amp, etc.
A circuit like the Fender AB763 found in the blackface reverb combos is a lot more complex and would require significantly more time to build than anything they currently offer. I think they are running at capacity now and you would have to wait until they could ramp up for more production.
More complex than the Overtones? Really? You may be correct, Nik stated just a little while ago that he completed a layout for a Vox AC30 but then shelved the idea as being too complicated.
Every single amp he current offers is either a copy or or derived from amps circuits that never had reverb, the Marshall, the Tweeds, the D*mbles, etc. To add reverb to any of his current models would completely change the amps tone and feel.
I certainly understand not offering reverb on Marshall, Dumble, etc. but to offer a line of Fender amp kits without a single reverb circuit? Fender is the industry standard for reverb! This is a glaring omission and not an unconscious decision.
Bottom line is that a Princeton Reverb is very high on my list for a future build and I still can't get it from my favorite amp kit supplier, Nik!