The official reason was the remove some of the old and more problematic units from tourney play. It was mostly targetting vehicles and artillery.
The major trouble makers were: DF DI Schmitt and DF Marksman combined with the DF Maxim. Dragon's Fury were
the KINGS of Tank Drop. On the Artillery side, it was pretty much all Sword Sworn Artillery. Unfortunately, this banning
pretty much removed Bansen's Raiders from the game, and, until Vanguard, left Liao with next to no usable units.
Remember that Sword Sworn and Storm Hammers had their killer Balacs, and and then there were teh Skadis and
Yashas and all those other less rugged, but every bit as dangerous VTOLs post Falcon's Prey..and Liao, until Vanguard,
had only one attack capable VTOL in the form of the the Shun.
Another reason, of course, was that before Falcons Prey, infantry and vehicles were lower point-valued. You could
field ALOT more infantry in the older sets. And there were people who would field all-infantry armies that were
extremely tournement viable. In fact, ALL the best vehicles and infantry(short of the Highlander Sniper Team) were
from before Falcon's Prey. If you used those, you would NEVER use a 'mech.