Well, because it is uncomfortable to accept that for Hindus, they always react with insults, some sort of other cope like OIT or whatever (which has been soundly disproved), they constantly demonize the sources that say that the invasion happened as, libpoopy or sepoy in nature, but the fact is it did happen.
And to be fair, this is something that happened more than 5000 years ago, and Indians are still unable to make peace with it.
Look, people of IVC were caught off guard, that doesn't mean that they are racially inferior just because they got conquered. Hell, a lot of early Aryan Systems were copied from IVC.
I think that the reason they don't want to agree with this as this might embolden the white people further more and make their "supremacy" more cemented into the heads of the people. That, it doesn't help that their are a lot of fair-skinned people in India who love kanging.
But it did happen, and it has to be accepted, either way, after that great mixing occurred, for at least a 1500 years. Something that made a lot of castes indistinguishable in appearance and decent bit of caste mobility also existed.
Although, if we were to go by today's measure the Jat, Med, and other castes have the highest Indo-European ancestry (Note: They are not the Vedic peoples, they were the Scythians).
And the other reason is that everyone deep in their mind thinks that to be brown is to be lesser, I mean, you can already see it on this imageboard, people often ask for each other's pics in order to mog someone through skin colour. So, the fact that Aryan Invasion (which it was) reminds them of white men ruling over darker men again. Something that is just not acceptable but it did happen regardless.