It's a hive city. The entire thing is one giant urban mountain so filled with twists and tunnels that needs to be retaken almost entirely by foot. A straight line March is admittedly unlikely. But there will be alot of cumulative walking involved with retaking a hive.

Plus grav chutes have limited fuel. And I highly doubt a regiment would be forced to wear them going into an environment  notorious for claustrophobic fighting conditions. Where honestly they are just going to be a hinderence going through doorways and tight spaces.