@ Chimetex.
I will give you rejection breakdown on the listed factors:
1, Travel History: Certainly you have not traveled out of Nigeria before and all of a suden, you wanna go for Diploma program in Canada. What is your motivation? A strongly worded reason should be stated in your study plan
2, Families: As a single man above the age of 21, you are considered an adult and a grown up who can live with or without his parents, also as a single man, you are considered a potential threat for immigration fraud by trying to get citizenship through contract marriage called (sham marriage), You poses the risk of overstaying. Based on the these facts, you need to go the extra mile to prove to the immigration officer that you cant do without you family, that you cant run away nor overstay. You need to show a strong tie to home country by means of real estate asset, family, employment or investment.
3, Purpose of visit: This is a very strong reason for rejection, if you apply 100 times, it will always be rejected for this reason: After a bachelor degree in computer science and with close to six years working experience in similar field which already made you a qaulified professional in the field, so why going back for diploma program? The immigration officer would either think you are insane or you just wanna use that to move to Canada and never return. If you ever wanna show you are a bachelor degree holder, then you should consider going for master degree but if you have a reason to go for a diploma program, then your bachelor degree should be thrown in the trash and never mention in your application. Now, its too late, its mentioned and they have record of your document.
Lastly, Personal asset and financial status: how much do you have? since when? whats are your source or proof of income? whats your current monthly salary? how did you prove to return after study? what do you wanna return too? Putting myself in the position of an immigration officer, I have loads of unlimited questions to ask which you have not been able to answer in your application.
For the above reasons, no amount of fasting and prayer will make CIC grant that study permit. So review, modified and re apply. Good luck.
Hope to get some fee for this detailed info ;D