In your opinion, why do these popular platforms act like this and completely ignore bitcoin?
The number of people who are willing to accept bitcoin as payment is smaller than the number of people willing to accept fiat payments.
Truth is that in majority of countries from where freelancers comes from, bitcoin is still in its baby steps, with some countries proposing an upcoming ban while some talking about regulations today and pulling out tomorrow. In such "unstable" situations, those who depend on freelance work for daily bread would rather go with the safer option, which is always fiat.
For them thinking about inflation and decentralization is more of a luxury than an necessity. Hence the lack in voice from their side as well.
And why doesn't a platform of the same kind, but using bitcoin, is able to develop itself to compete against the big ones?
There are such platforms, but they are much less popular. In fact whichever crypto-based project you try to develop when competing against a mainstream project, the former always ends up being a smaller one. Hard truth, it will like that for sometime.