Look: It may be a computer game, but think realistically. You CAN FRED a Colossus to have BFGreens on all its 62 turrets, but in real life, there is no way for even the Colossus to supply that much power to 62 beam turrets.
It took 20 years for the GTVA to build a single Colossus. Should the GTVA continue attempting to build supercapital ships, it would take them less time, albeit about 10 to 15 years.
The Colossus was never designed to take on juggernauts. It was a destroyer-killer, and given this purpose, it succeeds greatly. It wasn't a complete success, though (if its Fighterbay can be damaged, it isn't that good a warship). It still needs fighter cover in some aspects. When its hull goes critical, there had better be some friendly fighters around to protect it. In my incomplete remake of Their Finest Hour, I saw the following:
1. Sathanas jumps in, beams the Colossus down to 6% hull integrity;
2. Bombers disarm Sathanas' beam cannons;
3. Memphis jumps in from rear of Sathanas, takes ten minutes to destroy it;
4. Nephilims get sent out the moment Memphis jumps in, most of them make a beeline for the Colossus;
5. Nephilims bombard Colossus to oblivion because there was no fighter cover.
There we go. One of the underlying weaknesses of the Colossus is that it needs some fighter cover against waves of bombers that come in groups of three. In addition, because it is so large, it moves fairly slowly (although the Leviathan's even slower). Thirdly, if the Colossus takes 20% damage, it would take "months" to fully repair its hull. Finally, if the Colossus is put out of action (as seen in High Noon), it would be mostly useless to the GTVA.