The Colossus failed because the GTVA didn't expect to deal with anything bigger than another Lucifer. It was designed to deal with a bunch of small ships (i.e. clearing up NTFish rebellions and the like) rather than dealing with Sathanae.
Remember that, when it did use its full power, it almost melted its heatsinks. So its standard armament is crappy, yes.
The tactics of pooling all that firepower into one big expensive package are all wrong too. The Colossus can deal with some big targets, yes, but it can't be in more than one place at a time. So saving time, effort, and resources by building a ton of smaller, more expendable ships is the better way to go IMO.
Pack a bunch of extra firepower onto those smaller ships (cruisers, dropships, corvettes) and you get a powerful and mobile, yet cheap and expendable, war fleet. Thus, anticap cruisers are l33t.