I would have a look at who started using the word 'Mythology' to describe Hindu epics. Or more specifically, the question would become "Who first described Hindu religion in English language?"
What are the kind of cultural biases/nuances held by the people who described Hinduism to the European world? Were they affiliated to another religious system which treats 'non-believers' in a negative light? Does association with an Abrahamic religion influence your attitude towards people of non-Abrahamic religions?
What was the nature of interaction between English and India when Hinduism was explored by English? Was it an equal-to-equal relationship or a ruler-ruled one? What kind of views did other Abrahamic religionists have towards heathens?
Do they call their own religious texts as Mythology? Would one hear a Christian or a Muslim referring his/her respective theology as 'Christian/ Islamic mythology'?
Would the guardians/speakers of the language be obligated to modify the terminology when there is no real opposition to it? Would the word 'negro' be outlawed if Blacks didn't fight for it?
Just thinking about these questions answers most of the original question. There is no reason to rubbish one particular religion's self-described timeline as mythology when the same is not applicable to one's own.
TLDR => The basic reason behind calling Hindu epics as 'myths' is the inherent bias of the translators.