Weather related incidents are typically not covered by airlines in the US. While the reasoning of this rule whenever it started might have been understood, it has now become quite the excuse in airline ops to blame any problems on weather and unfortunately many passengers suffer from it. I've had a few times where weather was great at departure and destination, but "delays to the inbound aircraft" were stated as cause. (rolls eyes) And many do not realize the issue, like you mentioned, until after landing and seeing the other good options taken. I do think they would have the obligation to try and get you there faster if it was, for example, in the US, but as is I think UA have the only flights to Truk that make sense. Just stuck waiting I guess.
Although no help for you immediately in getting to Truk, you might look at any travel insurance you have or your credit card provides on purchases. Some credit cards are better about this than others. Also there are "trip in vain" rules, where you can just insist on being flown back to your origin and refunded your flights. I am not sure of the impact on weather in that case, though, as I have not come upon it myself although I travel a lot. You can also write to United, which you might choose to avoid after this, and they might give you a voucher for a future flight. Although due to weather they are not obligated, sometimes if you are kind and explain it rationally they will throw you a bone. I can't think of any liability they have for non-refundable reservations in this case, but I'll take a look.
Sorry for your trouble.
Edit to add: I do see that the visibility was low and clouds down to 900 ft with heavy rain at Truk. So while the aftermath is unfortunate, I do think UA was proper in getting the plane safely on the ground. These small island hopper routes would be nice on a 777, but honestly UA already has to get subsidies to make these routes economical in a 737, and they simply don't have the fuel to wait out big storms while circling.