Why didn't microwaving a stainless steel spoon set my kitchen on fire?
Metal has a lot of potential issues in the microwave (electric charge buildup + arcing, and microwave reflection). There are too many variables to make general statements like "such-and-such metal is safe" or "smooth objects are safe" with confidence, hence the sweeping guaranteed-safe blanket advice to not put metal in the microwave.

The reason it didn't cause issues for you is likely the fact that it was insulated in a plastic container (thus preventing arcing to the microwave walls / magnetron) and that you also had some water in there to absorb any excess reflected energy (and maybe electrical energy through physics that I don't understand, but don't quote me on that one). 

In fact a cursory search just happened to turn up this paper regarding sterilization of metal objects in a microwave, which mentions:


  Arcing back to the magnetron and damage to the microwave oven are prevented by placing a radar absorbent material within the oven and with proper insulation of the item to be sterilized


And you've got both of those things: A radar absorbent material (the water) and the insulation (a perfectly sealed plastic container).

The type and shape of your metal object is probably unrelated to the success of your accidental "experiment".



As an aside: It's worth mentioning that in the case where a microwave oven does catch fire, the NFPA mentions that 17% of those fires involved the housing/casing of the appliance itself as the first item ignited, where the danger is presumably excess energy reflected back to the appliance, rather than arcing (magnified by poor design or older microwaves that didn't have as good of an ability to absorb reflected energy). That's why, for example, the authors of the sterilization paper linked above included a radar-absorbent material, strange and unpredictable reflections off of metal can easily put the microwave under uneven energy loads it wasn't designed to handle (see also thermal runaway).

And of course, there's also issues with defective appliances or design flaws as well, e.g. that old GE spontaneous microwave fire lawsuit. While the causes of those fires aren't really related to the OP's situation, the point is making blanket statements about metals and such is further complicated by the potential for poorly designed or defective appliances.

Also, this answer isn't intended to say "insulation + water = always safe", it's just intended as an explanation for your specific experience. Even "safe" is flexible: A microwave fire doesn't necessarily lead to a kitchen fire or a burned down house or an injury, e.g. I suppose you could call a microwave fire hazard "safe" if you were standing there on the ready with a fire extinguisher and protective clothing.