First off if you read the books you should know the answer to this. Slipspace drives are as carter said the single most expensive pice of equipment made by man. Also if you remember in the game Kat suggested to youse the slipspace drive in lieu of a nuke because the unsc had already used up all of the nukes on reach. If you do a little research into halo cannon you'll find that the unsc use a lot of nukes some like the NOVA a 1.2 Petaton nuke which can destroy an entire planet. In conclusion a thermonuclear device is cheaper to mass produce, easier to use and much more effective. The only reason they used a slipspace drive is because they didn't have any nukes.

Secondly, it's not as if space combat in halo is even marginally realistic. The decks in the ships are built the wrong direction (they should be perpendicular to the axis of thrust) and any crew standing around in the ship when it accelerated would be turned to red mush on the wall. There are also no acceleration couches, which are necessary for the crew to survive high G maneuvers. The fact that halo occurs 550 years in the future makes the weapons available to humans looks like sandbox toys. I mean REALLY! The best armaments they have are RAILGUNS! Shit, 100 years in the future we will have a plethora of directed energy weapons including atom lasers, X-ray lasers, gamma lasers, singularity accelerators, and god knows what other planet killing devices. The fact that a human civilian half a millennia in our future could lose to a bunch of reptiles using PLASMA weapons is utterly ludicrous. And where are the hyper fast missiles, accelerated at 100 Gs and launched in massive swarms to overwhelm enemy point defenses. As a matter of fact, ALL weapons, including infantry weapons are RIDICULOUSLY primitive. Even the covenant are pathetic. Even the forerunner weapons pathetic! The forerunners, a galaxy spanning civilization dead for thousands of years somehow only manages to arm it's operatives with SUPPRESSORS! NO! The infantry weapons available to the promeatheans would -blam!- everything the UNSC could throw at them. Every single forerunner weapon should be a 1 shot kill to ANYTHING!

Did you hear Carter? It's the single most expensive piece of of equipment made by man. Why don't we make gold plated bullets 'cause gold is sorta hard, and we can better see the bullets? Cost effectiveness, that's why.

Your life sucks.

noble 6 was destined to die

Slip space drives were said to be prohibitively expensive. You could either build one slip space missile that, if shot down, would do nothing, or you could give a war ship the ability to operate in multiple solar systems/flee the covenant.