As we know BullGuard is using Bitdefender AV engine then why people will buy BullGuard instead of Bitdefender?
Bitdefender Globally ranket No.1 but BullGuard is No.10 why?

I will first answer with a question: Why do some people drink Pepsi and others Coca Cola (Coke) instead?

Each security suite on the market will have ups and downs because each company has it's focus on something else. For example, BullGuard focuses on being as light as possible and as user friendly as possible. This is why we develop BullGuard depending on the feedback that we receive from our users.

Performance wise, all security suites remove the infections that can compromise a system, such as "flame" (since one of our users mentioned it). It is highly unlikely for a home computer to get infected with an infection that targets businesses though.
Home users do not have highly confidential data on their computers. They have pictures, games, movies, music and maybe some school/work data. A home user doesn't want to feel like they have a security suite on their computer, because they need to be able to use their multimedia and plug and play devices and go on with their work, without hindrance from the antivirus/suite.

This is why, you should read performance reviews when you pick the security suite you want to use, but you should also try the suites for free by yourself before making a purchase.

I have found that Bullguard is cheaper then Bitdefender.

I can purchase Bullguard from a reseller for around £5.00. I have also noticed people selling older versions at a cheaper price. The product keys from these can be used for the latest version.

You are correct. The upgrade for the latest version is always free of charge. This is why you can purchase an older BullGuard CD and still get the same service and support from us.
Whenever you purchase anything, you should look for the best available offer.