system
Given the following post, respond as if you're commenting on Reddit:
human
Why are designs of the Hubble Telescope and James Webb Space Telescope so different from each other?
gpt
The HST was designed according to the limitations of spacecraft and telescope engineering and technology at the time. It uses a monolithic mirror which shares a fundamental design heritage going all the way back to Newton's first reflecting telescope from 1668. It's essentially just a big chunk of glass ground and polished to a very precise optical shape (ahem) and coated with a highly reflective surface. The observatory itself is basically a long tube (containing the light path for the optics) mated to a box containing the instruments and the spacecraft components (attitude control, power management, etc.) It's a fairly simple and straightforward design that was originally fleshed out in the 1970s.

JWST takes advantage of several innovations that have happened since the HST were built. Firstly, in the late 20th century the ubiquity of computing transformed the construction of large telescopes. At the time the Hubble was launched in 1990 the largest workable ground based telescope was the Hale telescope, with an aperture of 5.1 m (200 inches) that had been built in 1948. The problem with monolithic mirror telescopes is that you need the glass to be strong enough to hold the shape of the optical surface to within a fraction of a wavelength of light, across the entire surface, no matter what direction you hold the mirror in. This means you need extraordinarily strong materials to counteract the natural "sag" of solid materials in Earth's gravity, and that means you need very thick and very heavy mirrors. The Hale mirror used an innovative design (for the time) which kept the mirror weight at just 20 tonnes (not including the metal support structures also required to hold it up).

Creating a larger mirror than the 5m Hale in the same fashion would have required such tremendously weighty and massive mirrors that it would be infeasible to build or operate them. And that's where computers and digital imagers come in. If you segment a mirror up into multiple pieces you can still obtain perfectly usable images from those segments together but only so long as they are positioned with sub-wavelength precision relative to each other. For most of the 20th century doing that sort of thing was not possible, but starting in the late 1970s technology to do just that was developed. Each segment was mounted on a structure that allowed extremely precise control its position along with digital sensors. The segmented telescope is pointed at a bright star and a live digital imagery from the telescope is collected. At this point the image will likely look like multiple overlapping images (i.e. "seeing double" except perhaps seeing 6 separate images, or more). One mirror segment is chosen as the "master" and each other mirror segment is adjusted in turn to align its "double" image with the master until only one aligned image remains with all of the segments aligned to one another, to form one effectively much larger mirror. During observations this alignment is continuously maintained.

The advantage of this is that the combined weight of all the support structures for the mirror segments is much, much smaller than would be the weight of a monolithic mirror and its support structure, making large mirrors much more practical to build. In 1979 the "Multiple Mirror Telescope" came only, built with 6x 1.8m mirrors that together operated as a single effectively 4.5m diameter mirror (the 3rd largest in the world at the time). In late 1990 the first Keck telescope was finished and saw first light, this was a more advanced segmented/multiple mirror telescope design which contained 36 hexagonal segments to create a 10m diameter telescope, vastly surpassing the previous record holder. Since then, 13 additional telescopes with mirrors larger than 8m have been built, vastly expanding our astronomical observing capabilities in a fairly short period of time. Another key element here is active control of the mirror's optical surface using actuators that are placed behind a thinner, flexible glass substrate. Mirrors of that design are even lighter but require a bit more trickery to maintain the right shape.

On top of that you have many years of experience building satellites and spacecraft. You have years of experience building and operating ever more advanced astronomical instruments (both on the ground and in space, especially on HST and other space based observatories). You have years of experience building and operating IR telescopes, especially Spitzer.

So you have a situation where launching a monolithic mirror based next generation space telescope would be impractical and infeasible. A 6.5m diameter single piece mirror would weigh 10s of tonnes, and that alone would eat up all of the available launch capacity. Besides which, you couldn't fit such a huge observatory inside any rocket that is currently operating. But fortunately by the 2000s segmented mirrors and active wavefront sensing and control technologies which enabled potentially extremely lightweight large mirrors had become much more mature. And spacecraft construction techniques gave much more confidence in being able to "deploy" components of a spacecraft in space after having been folded up for launch. Essentially all modern commsats do just that, deploying not just solar arrays but also antenna and other structures.

Additionally, once the design of JWST focused on infrared observations, that drove the design increasingly toward one that made use of sunshields and cryocoolers. Partly precisely because advances in mirror technology unlocked the potential for very large mirrors to be deployed with very little weight that set JWST down the road of being a spacecraft which folded up for launch and unfolded once in space, as mass became less of a constraint but volume (within the launch fairing) became more of one.