(a) it will be observing a different part of the spectrum to HST;
(b) it will be in a different location, L2 rather than LEO;
(c) its optics will be shielded by a sun shield;
(d) some other factor ?
It's mostly (c) but that only works if (b) is true.
By being in a halo orbit around the Sun-Earth L2 point, the Sun, Earth, and Moon are all in roughly the same direction, and so can all simultaneously be shielded by a single shield.
In this answer I talk about JWST's planned orbit and include the diagrams below taken from NASA's Stationkeeping Monte Carlo Simulation for the James Webb Space Telescope
In this comment @AnthonyX says:
I think the answer to this question is in the NASA FAQ linked in the question. The telescope needs to be kept very cold to observe faint objects in the infrared. It has to keep Sun, Earth, and Moon constantly on the other side of its heat shield to stay cold enough. The closest place that meets those requirements which is also an orbit (minimal station-keeping fuel) is Sun-Earth L2.
I've added links there to put the quoted comment in context.
But JWST needs more than optical shielding from stray light! All of JWST needs to be kept very cold because it is an infrared telescope. Luckily, as long as the sunshield protects it from being warmed by strong sunlight and thermal infrared radiation from the Earth and Moon, JWST telescope itself (mirrors) can keep cold by radiating its heat directly into space.
There may be some additional refrigeration for the infrared sensor array, but the optical surfaces cool themselves by radiation.
I also think that JWST could have a tube if it needed it. It doesn't matter that it's huge and folded, if a tube was necessary, an "origami" tube could have been invented that would fold out somehow. Need of a tube wouldn't have been a show-stopper for the JWST, look at the complexity it embraces by rolling out all of those layers of thermal insulation!
above 1: Source, above 2: Source: Status of the JWST Sunshield and Spacecraft , found here. below: Source: Stationkeeping Monte Carlo Simulation for the James Webb Space Telescope