Not having a guarantor ready to go.
Landlords have tough criteria everywhere, though it is definitely the worst in New York City. The typical income requirement is 40x the monthly rent. Meaning, if you want to rent an apartment that is $2000, then you'll have to prove an income of $80,000. If you were to have a parent/family member cosign your lease, then they would have to make 80x the rent, or $160,000.
Most people and their families have a very difficult time meeting these standards. For the parents/family members who do meet the insane 80x income requirement, the majority feel violated with the amount of paperwork they have to submit on behalf of their child and get frustrated. In the last few years a new type of service has been popping up called a "rental cosigning" service. The one that is most trusted would be Insurent.com. They act as your "institutional" guarantor and will co-sign your lease for a fee. What is great about them is that they are already accepted by all the major landlords (NYC, Chicago, Boston, DC, and soon more) and their income requirement is only 27.5 x the rent. That same $2000 studio income requirement is now a more palatable $55,000. They also require far less paperwork if your parents would rather use them then be on the lease directly.