Well, beng has pretty much told the story. So here's my reasons.
1) First of all I like café racers, I also find them quite comfortable with my build being short legs and long arms, maybe I'm closer to some of our evolutionary friends than others
2) The Norton featherbed framed bikes, IMO are the some of the best looking bikes of all time, especially the Manx and Dommie racer.
3) I could never afford a Manx, and even the twins are expensive, in fact these days anything with a featherbed frame is expensive.
4) I actually think that the inclined Commando engine looks better in a featherbed than any vertical Triumph or Norton lump, but the engines are harder to find and more expensive than Triumphs, they also need a little more work to run as they should, crank rebalancing etc.
5) With a Triumph lump (at least in the case of a Bonnie) you get twin cams and twin carbs, it probably makes sod all difference in the real world, but they look cool.
6) Now, why choose a Triumph pre unit or unit motor? Well, I think the pre unit looks better, but it doe's require a fair bit of work to get it up to the same level of reliability as a unit, machining out the timing case for an oil seal, fitting a later crank etc. The unit on the other hand, is build up as you want, but with minimal engineering work required, they are also plentiful and cheap. Finally, in my case I already own a unit Triumph, so I have all the specialist tools, experience and spare parts.
7) The great thing with a featherbed frame is you can fit almost anything you want in it, so if I ever get bored of my Triton I can always fit a Norton, BSA, Vincent etc. lump in there. It can also be changed from a café racer into a tourer if I want, in fact it should be the last bike I ever have to buy, I just need a couple of engines
Finally, the name's cool !
All the best