A small error...

[Judge_Deadd]

Well...
computers require Mathematics and Electronics, but if you have Elec. you also have Maths. Look: Mathematics->Physics->Magnetism->Electricity->Electronics. I guess Sid couldn't stand a modern tech with only one prerequisite.

 

[Matrix]

He just wanted to be consistent in having every tech require two others, I think.

 

[ivanbuto]

It could be that you didn't develop Mathematics at all, and acquired other technologies by stealing from enemies. In such case, simply having electronics would not allow you to develop computers.