Quoted:
Multiple threads in GD about it.

Jupiter is either too small, or too D-poor to achieve the same level of burn achieved by other near-mass kin brown dwarf stars.

Stellar characteristics

TRAPPIST-1 is an ultracool dwarf star, of spectral class M8.0 ± 0.5, that is approximately 8% the mass of and 11% the radius of the Sun. It has a temperature of 2550 K and is at least 500 million years old.[4] In comparison, the Sun is about 4.6 billion years old[16] and has a temperature of 5778 K.[17]

Owing to its low luminosity, the star has the ability to live for up to 4–5 trillion years, meaning that TRAPPIST-1 should remain a main sequence star when the Universe is much older than it is now, when the gas needed to make stars will have been used up.[18] The star is metal-rich, with a metallicity ([Fe/H]) of 0.04, or 109% the solar amount. This is particularly odd as such low-mass stars near the boundary between brown dwarfs and hydrogen-fusing stars are expected to have considerably less metallic composition than the Sun.[citation needed] Its luminosity (L?) is 0.04% of that of the Sun.