The antenna panels caused bright flares due to a combination of two factors:
- antenna panels were oriented at a (roughly) 45 degree angle with respect to the nadir. This positioned them perfectly to reflect sunlight down on to the Earth at dawn and dusk, when the spacecraft were in sunlight but it was getting dark on the ground. Solar panels are usually articulated or rotated to face the Sun as the spacecraft rotates to face the Nadir. This means they are not likely to reflect towards the Earth very often.
- Iridium antenna panels are flat and metallic shiny; highly reflective an specular. A huge amount of work and cash has gone into minimizing reflectivity of solar panels because their job is to collect and absorb sunlight.
I don't think there will ever be other satellites with these wide, flat Iridium antenna panels again. They are really characteristic of the old Iridium satellites.
The first images with the shiny metallic surfaces are the old Iridium antenna panels taken from What goes into an Iridium Flare prediction model besides the ephemerides?
The last image is the new Iridium Next satellite and they've moved the antenna array under the body of the spacecraft and it's no longer shiny and reflective. That's from this answer to the question How many communication antennas does a satellite need if it belongs to a constellation?
Old and Busted (original Iridium)
below "View of one of the Main Mission Antenna. The hinged base is on the right side." Cropped. From here.
below cropped image of a first generation Iridium satellite, showing a bumpy and diffusely reflecting metal antenna panel of the type responsible for producing the flares seen on Earth. From here.
New Hotness (Iridium Next)
above: Conceptual drawing of Iridium-NEXT from Harris.