Once you are married to anyone, you immediately assume the titles which the different relatives have to address you with. Remember, you have to get married before you get these title(i.e. you're not a brother-in-law, unless you are married and then by law you are a brother, although not a blood-brother).
It is not necessary that your marriage pre-date the birth of any of your 'gained' relatives. For example, your step-son will call you step-father even if he was born before you married his mother.
You have noticed a very peculiar aspect of English vocabulary. As rich as it is in comparison to many other languages, due to its almost creole history, it really is impoverished in comparison to other languages in kinship terms.
But 'why' is always a difficult question, especially when mixed with cultural questions. There are the difficulties with Sapir-Whorf type explanations: both language restricting thought on one hand and the number of Eskimo words for snow on the other.
Does the lack of kinship terms reflect the cultural lack of warmth and caring for relatives among English speakers, that is not caring leads to the loss of the words (which etymologically do exist in the ancestor languages), or did the arbitrary lack of kinship terms contribute to the crumbling of English family values?
Any direction sounds much too tendentious, too judgmental, and requires too much unjustified and biased assumptions to choose.
The comparative lack of kinship terms does ask for an explanation but one backed up by linguistic and anthropological and comparative research. The only source that comes to mind is Levi-Strauss's 'The Elementary Structures of Kinship.' (primarily anthropological but as a by product a number of examples of kinship term systems.
English isn't alone in having relatively few kinship terms. Some other European languages have only a few extra (French, German) and some languages really only have names for their clan and generation (anybody of one's biological parents' generation might be called something like 'uncle' or 'aunt', even one's birth parents).
Having no definite answer to your question, I can only say beware of making cultural inferences based on restrictions to languages. Some languages have grammatical gender and others don't, but that doesn't mean the ones without can't recognize the sex of other people.
Best Answer
It's a matter of culture and how close you interact with him. Technically, I dont think he is your uncle anymore. He is a new guy in a separate family and doesn't have to do anything with being your uncle. But if you are still in close contact with your aunt and want to stay with her in good terms, why not. I'd call him uncle. And there's nothing wrong with it. I have seen people calling strangers uncles just because they know their father.