Interesting point, hopefully these links might help with regard to placement of effects and their respective impact on tone.

http://proguitarshop.com/andyscorner/fx-loops-explained
http://www.premierguitar.com/articles/Effects_Loops
http://www.bearfootfx.com/amp-effects/

Not withstanding the information in the links above, The amp models pin the 11 rack are primarily classic 'vintage' models (particularly prior expansion pack), that did not have an FX loop, so it kinda makes sense why there is no option to place the FX loop between the pre and power amp.

Also, IMHO the 11 rack (despite any claims by AVID!) is really squarely aimed at trying to create 'studio tones' to allow 'in the box recording'. So again, it makes sense that effects (time based) would be added after the 'miked amp' tone, just as they would have been added to those classic amp tones in the studio.

You could argue (argue away ) that when the modern amp models were added, that they should have also added the option to place the fx loop between the pre and power section (with options for series and parallel routing).

But we all know we are kidding ourselves, thinking that AVID would give us the fully baked product what we truly want(ed) even if it was in slow increments...

Having said all that, the only people who can truly answer this question, are the original design team, wherever they are now! In the mean time enjoy the 11 Rack for what it is rather than what you wish it was (just like those classic amps/fx pedals!)