I mean this is like asking why the Romans named Gaul "Gaul" when according to Caesar, it was also inhabited by the Aquitani and the Belgae. Brittonic Celtic speakers would have been the population that Mediterranean traders and merchants had the most contact with (tin mined in Cornwall was shipped to the Mediterranean while wine was exported to Britain) while the Irish, despite possibly being in contact with the same peoples, probably just didn't have such sustained contact to warrant lending to the place-name.

Giving the separation of p & q Celtic and later Gaelic and brythonic language ,why would the greeks call these isles to be inhabited by the britons when one group was Goidelic or Gaelic

This is a very old-fashioned view of the historical development of the Celtic language family. Since the 1990s, historical linguists like Kim McCone and David Stifter have argued the position that the distinction between P and Q Celtic languages is irrelevant to their configuration in the language family, and that both Brittonic and Goidelic languages emerged from a common "Insular Celtic" parent language separate from the Continental languages, rather than representing two successive waves of language spread.

Initially, the Proto-Celtic language seems to have lost the Proto-Indo-European P consonant explaining its absence in early Old Irish for example. However, some later languages like Brittonic and Gaulish eventually restored a P sound by transforming the Proto-Celtic Kw into a P. This does not actually imply common ancestry though, because we know for a fact that Gaulish only underwent this transformation comparatively lately or else demonyms like the Sequani would have been pronounced and transcribed as something like Sepani.

In fact the gulf between Insular and Continental languages is much larger than any dissimilarities between Brittonic and Goidelic. Continental languages remained much more faithful to Proto-Indo-European in terms of retaining its case system (rendering languages like Gaulish very similar to Latin for example) while Old Irish's case system is fairly diminished, and Brittonic languages have lost it completely. As well there is the peculiar Insular verb-subject-object word order which is not found in Gaulish or Celtiberian. Finally there is the system of initial mutations (the mutation of the initial syllable of a word depending on its context in a phrase) used in both Insular groups which appears to have been a novel development as it has no parallels in the Continent.

also its important to note that the cruthin are not considered Qritani the irish clearly made a distinction between the two groups

I have doubts about this sentence. While the Cruithin demonstrably adopted Irish cultural habits and language by the time of recorded history, there's no reason to believe why they weren't originally Britonnic settlers from Strathclyde in Ulster. Etymologically, the name just means "Briton", but underwent a transformation thanks to the absence of P in early Old Irish, which was replaced with a labial-velar Q. The term is also used in some Irish annals to refer to Britonnic speakers in Britain in several instances.

So the reason why the Greeks and Romans called them the "British Isles" probably just has to do with the fact the the Britons were the ethnic-linguistic group that they were in contact with the most. You have to remember that from the Mediterranean perspective, these islands were some desolate crags at the end of the earth; they probably just didn't care enough to undergo a substantive survey of its inhabitants demonyms to establish a "politically correct" place-name.