Ogrork the Mighty said:
Because it's not meant to be a mass combat rulesbook. There are plenty of other systems to achieve that end. This particular sourcebook is meant to show how a party of adventurers can fit into a campaign involving a war, rather than how the war itself would play itself out mechanically.
To add to that:
Before reading HoB, I was constantly on the search for some mass combat system that felt both playable (not too clunky) and detailed at the same time. None of the available systems that I tried out (Fields of Blood, Slavelords/GT, Cry Havoc, Warcraft Mass Combat) really did it for me - but Heroes of Battle convinced me, that, actually, I don't
need mass combat.
Really, what gaming is all about, the bottom line, is the adventures of the PCs, not a battle simulator. HoB puts forth the premise that the DM can plan the battle beforehand, and then give the PCs the chance to influence it at certain points to achieve different outcomes, depending on what they do. Railroading? I don't think so. It's a necessary simplification that lets you concentrate on the encounters that your PCs have, while the battle rages all around them. If you really want to do some randomization, though, you might consider using Slavelords' system in conjunction with HoB to great effect.
Long story short: Great book, 4/5 content, 4/5 art (not a single 'meh' piece this time - great stuff by WAR). I love it, and I'm glad that I spent my money on this and not Champions of Ruin. It's definitely a DM's book, though (which, in my book, is a good thing).