you certainly have a lot to learn nayani, no person is under obligation to stick only to the major currencies, spreads are typically or noticeably smaller when compared to exotic currency pairs though.
The simple answer for some currency pairs is that only the government can change the price of some currencies and they don't allow trading with digital amounts.
For example in Taiwan you can have a bank account with USD, HKD, GBP, AUD, CAD, SGD, CHF, JPY, ZAR, SEK, NZD, THB, EUR, KRW, VND, or MYR, but can only deal in cash for PHP, IDR, and CNY. That's the law, not the convention.
You can also look at how you can trade crude oil if your account is in the UK, but you can't if your account is in the US or Taiwan (dunno where else).