I think because the meaning of the interjection "haha" has acquired an additional shade of ridicule or sarcasm. Therefore, to avoid ambiguity, resort to "ahah". The latter is a more accurate iconic sign for real, sincere laughter ("aaaaaa-hahaha) and is associated, moreover, with another interjection" aaaaa ", which also conveys laughter in writing."
In general, the expression of emotions on the Internet using both smiles and interjections has existed for a very long time and, given the high dynamism of the modern language in the Internet, many words and jargon undergo very rapid metamorphosis. But it is also worth noting the heterogeneity of the Internet contingent, as a result of which in different communities different trends (including slang and communication style) can be very different and develop in different ways. Therefore, it is difficult to say abstractly that some word is fashionable now, and some is not. And if we consider that for the expression of laughter there is already just a great many neo-interjections: from BGG to muahah, in this case it is more appropriate to speak not about fashion, but only about specific cases when the author chooses onomatopoeia that suits his emotions