Because with [radi]us the stem is "radi" but with [vir]us the stem is "vir". These words are from the same type (second declension) they both receive an -i affix in plural, hence radi + i = radii, vir + i = viri. Latin being Latin there are an awful number of exceptions but this is a somewhat general rule.

Put it like this: Grammatically speaking, the plural of virus is viri. Putting it into plural might or might not makes sense. Personally, I don't think that using plural for collections in Latin is a very big sin given that this is very common in classical Latin texts.

One example of this can be found in the famous introduction of Aeneid (I.1 "Arma virumque cano...") lines 31-32, where Virgil is using the plural form of the word "sea" (mare, plural: maria)

"multosque per annos / errabant acti fatis __maria__ omnia circum" - "for a number of years, driven by fate, wandering around on the seas"

Virus is Latin for poison. It's a mass noun because it denotes something uncountable (not in the strict mathematical sense, but in the how the hell do you count poison sense). As far as I know, there is no Latin plural form for virus.

Second declension singular nominative nouns end in 'us' and their plural form end in 'i', but fourth declension singular nominative nouns also end in 'us', but their plural form still end in 'us'. Also, like in every language, there are funky exceptions to these rules, like second declension singular nominative nouns which are neuter rather than masculine, but still end in 'us' rather than the normal 'um'. Moral of the story, don't assume that the plural of word ending in 'us' is 'i'.

It's also been about 8 years since I've taken Latin, so take that into consideration before someone goes all Life of Brian on me.

Interestingly, English words with irregular forms which are infrequently used often revert back to regular forms. The 'why' is simply because people prefer to say "viruses," either because they forgot the irregular plural form or because they prefer how a regular form sounds.

And furthermore "sands" is still perfectly legitimate, even though "sand" may be inherently plural (the "sands" of time, different "sands" of the world).

If the plural of goose is geese why is the plural of moose not meese?