if multiamplification is such an obvious advantage, why don't I hear more ..... Just wondering.
Knownothing,
I think you have nearly answered your own question...a lot of it is established marketing psychology whilst some of it is historical in origin. The High-end audio industry is more akin to the fashion clothing market than pure engineering. Industrial design is an important aspect of most products and may often determine their success - after all these things sit in people's homes.
There is also a "value network" established between individual component makers that can be "matched" by the consumer/dealer to create unique combinations. You are obviously unlikely to hear an amp manufacturer or a passive speaker maker endorse Active designs, as this is promoting a competitor's product...and currently there are far far more individual component makers than there are "Meridians".
The reality is that individuality is also extremely important to high end customers and the existing approach allows for mix and match for clients, just like women's clothing: women dread the thought of turning up to a ball wearing the same gown! Funnily enough, men are comfortable in these situations wearing identical tuxedos....not so, however, for audio!!! The value network is similar to Apple iPod success...the accessories market is a big part of why it is attractive to sell iPods...people keep coming back for incremental purchases.
Active designs have had somewhat greater success in professional circles where transparency and how a mix/master translates is more important, but it has been far from a cakewalk; studios also like to differentiate from each other - a kind of gear arms race - my gear is better than yours - it can't be better if it is the same - so most often "custom" designs are made, even with active speakers. However, in this market there is a greater need for engineered accuracy to help remove a little of the guess work when an audio engineer works in one studio or another (often with a new combination of gear). Although an alternative, still popular, is for studios to simply stock or rent dozens of various reputed speakers (all diffeent sounding) simply to allow clients to hook up what they are already familiar with for near field monitoring.
As for the "obvious advantages" of Active Speakers...yes, purely from an engineering perspective, the advantages are indeed quite obvious.....but most people regard integrated multi-amped active speaker designs as too narrow a "straight jacket", restricting the freedom to mix and match for sound, which is the main reason many people pursue this hobby - it is one of collecting, experiencing and trading...and dare I say it fashion too!