Why is rushing B a famous thing? please tell me i want to know lmao

Do you know CS:GO is, and do you play it?

Rushing 'B' is probably because the B bomb site is usually easier to take during pistol round. The reason is that on most maps the route to B bomb site has tight corners as oppose to A site, which has long distances.
On long distances, the CT's pisol USP/P200 is more favourable as they can easily one-shot headshot (during pistol/if not wearing headarmor). If both a Terrorist and Anti-Terrorist shoot each other in the head, the CT
will instantly kill the terrorist whereas the terrorist would only tag the CT (amount based on distance).

Thus B is favored, as if a CT is close enough you can one shot them in the head. Generally 'B' is easier to take unless you, and your team, are good at pistols and/or have a plan to take it other than rushing.
Russians are stereotypical scrubs at CS:GO. If you aren't good at the game, it's only logical to take the easiest route to a bombsite. By rushing, you also have overwhelming power with your team as it will be a
five versus two/three in most cases, and you will have taken over the site before the rest of CT' come to re-take.