Just curious — Crown xls 5202 amplifier

I was watching a YouTube video on the Crown xls 2502 amplifier.  It is rated at 400 wpc at 8 ohms and only costs about $650.  I was curious as to what guts this thing has underneath, to create that kind of power in a slender cabinet.  But when the reviewer took the cover off, I was surprised at how little there was inside this thing.  There were 4 (what looked to be) filter capacitors, a small transformer, a small coil that looked to be copper and a couple of smallish heat sinks.  It didn’t look like anything that would produce that much power.

Do you know how this technology works?  If it is able to produce so much power in a small space, why don’t more manufacturers build their amps this way?

(The reviewer noted that such amps are used in commercial applications and this amp was noisier and did not produce as full a sound as some other high-end amps.  Still, I would think a lot of guys would want that kind of power for such a low price).