Because melee AI scripts override one another.
But afaik some scripts can be mixed. For example, you could try "campaign xxx" and "expansion yyy".
However you can't mix races at all. The computer will only properly use AI scripts that fit his race setting or are independent of race.

Thanks I have just a couple more questions to make
sure I'm understanding this fully...

Someone on here said that AI scripts with the Expansion predicate meant that that script is for a secondary town, however the help guide in m Campaign Editor states that that only means that that script will use the new units available in Brood War. ??

Also: So does that mean I can have only one computer opponent in a map, or can I do multiple opponents, as long as they are of the same race? And if the later is true, can I run multiple AI scripts that are the same, or do they have to be different (but still for the same race).

I hope my questions make sense lol. I only started playing Starcraft a few months ago, and I've only been using the Campaign Editor a few weeks hahaha. So I really appreciate the advice!

An expansion script is what the name suggests: The computer expands to another mineral spot to get more ressources for more units. What units he will build and when is still governed by the main script.
So no, it's not a 2nd player.

I guess you may have checked the `Random start location` in forces panel right? Try unchecking it.

BTW, I know it's been 7 years so this reply is actually for someone who might came across similar issue in the future

Lol nice necro. I just had this same problem. The computer player has about 14 serf units and was terrain. After swapping the ai computer with someone who had 0 pre placed units it worked. You have to also make sure that they can do everything (build all units, upgrades, tech. Etc). If they try and make A marine but can't because it's disabled the script will halt and not process anything else.