Let us stipulate that many people say "nucular," even the educated. But why? Can't they see how it's spelled? That won't work, though; lots of words in English are inconsistent with their own spelling.
Another explanation is found here:
One reason, offered in a usage note in the American Heritage Dictionary, is that the "ular" ending is extremely common in English, and much more common than "lear." Consider particular, circular, spectacular, and many science-related words like molecular, ocular, muscular.I don't think that will work, either, because those who say "nucular" for nuclear are also naturally prone to pronounce nucleus as "nuculus," and in this case, I don't think "nuculus" is a back-formation from "nucular."
The real reason, in my humble opinion, lies in the nature of the phonetics of this word. The soound /l/ is a sonorant, as are /m, n, r/; all the other letters are called obstruents. Sonorants are peculiarly prone in English either to elision (such as "libary" for "library") or else they like to have helping vowels around them, especially when they appear in a cluster, as in nuclear. Consider also the pronunciations "athalete" for athlete, "realator" for realtor, "filum" for film, or "libarary" for library. All have helping vowels between the sonorant and the other consonant. I think "nucular" is another example of helping-vowel insertion.
This process is called anaptyxis. And it particularly interests me because it happens a lot in the Semitic languages. In Aramaic, for example, the word מדנחא, madnecha, the East, often is vocalized madincha, with the anaptyctic vowel between the dalet and the sonorant nun. Another example: מזרקין, mizreqin, "bowls," is often vocalized mizirqin. In certain dialects, these pronunciations become the norm -- the "proper" pronunciation.
There are tons more examples from Semitic that I could cite; tomorrow I plan to talk about a particularly interesting one. But for now the point simply is that such pronunciations are a natural part of language change; and it's not worth going ballistic about -- or should I say nucular? In a few years, people could laugh at you for saying "nuclear."