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.