One reason is when there is a currency translation involved and a fee for that.

Another reason is when one of the two parties lives in a country where there is an "official exchange rate" and a "street exchange rate" for their currency. In this case they may get 10x the value by using bitcoin. PayPal and credit cards go through the banking system hence are uniformly at the official exchange rates.

Of course in this case the authorities may well have made bitcoin illegal and for those very reasons.

This subject is prohibited at my country. It's illegal.

I think your thinking is a little flawed here. Why would a customer go "Paypal -> Bitcoin -> You"... when they can go "Paypal -> You"? Likewise for CC... why "CC -> Bitcoin -> You"? Where is the "value add" for the customer to get involved in Bitcoin?

If you're not offering any incentive for using Bitcoin, what is the point? Only people who already have Bitcoins will use Bitcoins to pay you... otherwise, they'll just pay with Paypal or CC.

Have you considered offering some sort of promotional rate for folks who want to pay by Bitcoin? Maybe a small discount or a free month or something like that... It would help promote the fact you have a new payment option and with Bitcoins current trend, probably would not cost too much in lost revenue as the gains in BTC value would hopefully offset it. People love "free stuff"™ and discounts etc... You can guarantee that if users can save a buck or three by using Bitcoin, they'll do it!

If you have a customer base who have opted in for mailing... Highlight the new payment option in an email campaign.

Social media?

I mean, this stuff is basically marketing 101... Just putting a "pay with Bitcoin" button isn't going to do much... You need to basically have the equivalent of giant flashing neon signs highlighting it... "People"™ are lazy and "blind"