Seven of my last eight amplifiers have been monoblocks. With their dedicated (not shared) power supply, monoblocks typically offer the advantages of superior channel separation without crosstalk, improved soundstaging, absence of inter-channel modulation, and increased dynamic headroom. They also facilitate the use of short speaker cables, at the expense of longer ICs and PCs. However, monoblocks using a bridged load design can be noiser and are certainly less effective (or even problematic) when driving low impedance speakers.
To your question, I have found monoblocks to be quite prevalent although the move to more compact systems (by some) would certainly benefit by the use of stereo amplifiers. In addition, all things being somewhat equal, stereo amplifiers are less expensive (per watt of power) due to the absence of redundancy of expensive parts such as transformers and metal chassis. Therefore, they may be an easier product for manufacturers to move.

