"If they are poor people who earns less than 1 lakh, i dont find a reason why they cant directly go to bank account and exchange money in next 1 month."

Perhaps because they can't wait a month before their next meal or fuel for their scooters or for household expenses?

"Now if those people are blaming govt for banning those notes, i find it funny."

I find it funny that some people find it funny that 85% of the currency bills in circulation are invalidated overnight. Maybe it's funny if all diesel vehicles are banned overnight because they pollute a lot? Or if mobile phones are banned overnight since they are used by terrorists?