i heard many Cryptocurrency but why people mostly use bitcoin?
Because other crypto currencies are just good or available for trading but nothing else, while bitcoin has a wide range of acceptance online and offline too, why would someone use a currency or a digital currency that is not accepted anywhere? That is why people use bitcoin more as it is accepted or usable in more places.
to me most most of the people are comparing bitcoin with USD and the reason is that USD is one of the most popular fiat currency of the world and almost all the people of the world know about it and its price any time. some people also compare it with their own currency as to know the actual amount that you have in form of bitcoin.