OK, second try at this. Last attempt mysteriously disappeared.

So, it appears I was somewhat wrong in my prior statement. There is one situation where tadpoles technically reach sexual maturity. I'll discuss that later in my answer though.

The first, and most common reason tadpoles fail to metamorphose is due to insufficient growth with normal developmental progress. In larval amphibians there can be a trade off between growth and development. They can grow larger at the expense of slower development, or develop faster at the cost of a smaller size. Growing big is good for surviving against predators, and post metamorphic survival, while rapid development is better in high competition and fast drying wetlands.

If the tadpoles don't reach a certain minimum mass to metamorphose, they die due to lack of energy reserves. Basically starve and or fail to develop properly. That being said, they can remain in wetlands as tadpoles for longer than their healthier cohorts. This would be especially noticeable in species with long larval periods like green frogs (Lithobates clamitans) and bullfrog (L. catesbeianus) that take multiple years to reach metamorphosis.

Second way tadpoles might fail to metamorphose is a thyroid problem. Either lack thereof, or failure to produce sufficient thyroid hormone. This has been documented in at least bullfrogs (L. catesbeianus) and African clawed frogs (Xenopus laevis).

Here is where I was wrong to say tadpoles won't reach sexual maturity. Xenopus tadpoles,lacking thyroid glands, can fail to metamorphose, but still produce mature gonads with eggs or sperm. These are said to be the closest case to anuran neoteny. It should be noted that they've never been observed to reproduce and there are major issues that would likely prevent them from doing so. It's also been observed that while these Xenopus tadpoles have lived in labs for up to 8 years, any attempt to force metamorphosis results in death at the stage where the tail is resorbed. It's thought this is due to disproportionate tissue and organ growth.

The main reason these tadpoles that fail to metamorphose may die early is due to wetlands drying, lack of food resources over time, or predation. They are essentially big squishy biological water balloons with no way to escape a drying pond and few defenses against predators, and they are bigger target than other tadpoles. Aside from that it's possible they could live a long time, but they are also very rare. So, these are more or less an exception to the general rule.

Thank you for surviving my dissertation.

*edit: spelling and added some species names.