What are the conditions for a coin to be eligible to be staked?
The most important condition is to have that implemented.
While many altcoins/shitcois do have that, Bitcoin doesn't. Imho Bitcoin is way better without PoS/
And why are the returns on staking different on different exchanges?Is it simply due to difference in commission ,as logically the return on staking should be universally same depending upon the lock in period?
In the same way different banks in different countries give different interest rates for the money you deposit in different currencies, this is how I see whatever the exchanges and other businesses do.
You may also confuse staking with interest on deposits or with interest on lending some platforms may do, especially as they advertise their "products" in various manners.