Visitor Visa - B2

hi immihelp,
Sir,
already applied for tourist visa me and my mother,
i am a job holder ,worked for multinet group as a customer service executive,my salary is:17000,mentioned in the form,also mentioned the form tourism expenses will be bear by me.my bank statement was around 4 lac in BDT.my cousin brother also sponsored to me about tourist visa.

interview time:(20.11.13)
VO started asking question
1. asked for passport,
2.why u want to go to usa?
ans: pleasure & get together to my cousin and his full family also i would like to visit some famous tourist destinations.
3.what is your cousins name?
Ans:replied.
4.in which place want to go?
ans:washington dc,newyork,dalas,niagara fall.
5, how long u r going to US?
Ans:3-4 weeks.
5.how do u do?
ans:worked for multinet group as a customer service.
7.your salary range?
ans:my total monthly income 32000 ,from work station i received 17000 and as a private tutor i received 15000.
8.who will bear of ur expenses?
ans:that time i was nervous asked him again because i could not realize what is he asking?then he asking again? then answer me and my cousin both.air ticket i can manage it,and us expenses my cousin will be manage.
8.do u have ur bank statements?
ans:yes,given to him,after seeing that he input something in his computer.and checking my balance with carefully,after that told me sorry this time we can not approved ur visa,have some rules ,then give me a paper.there are basically have some rules that make sure i can came back to my home country.in my statement there is around 4 lac in BDT.

he did not even check my brother sponsorship any documents.even i did not mention that it was sponsorship for our visa.invited for christmas and newyear.pls tell me what was the fault about my application and interview session?