Short answer: white male citizens of the American colonies were absolutely able to keep and maintain firearms before the Second Amendment was passed. In fact, gun ownership was absolutely in place prior to the start of the Revolutionary War and dates back to the initial founding of each of the colonies. Many people owned guns and some of the trends are mentioned in a link to a previous question that I have linked below. I'm not sure why there is an assumption in your question -- the assumption that this statement implies that the right to keep and maintain arms was being infringed, but over-all across the colonies, it wasn't. It was something that was debated during the Constitutional Convention in 1787 (limiting gun ownership to specific people) but it was shot down.

Ultimately, we need to answer why they included this amendment in the Bill of Rights and surprisingly, it has much more to do with what was happening after the Revolution rather than before. There were many different debates that were taking placing between 1787 and 1790, and some members of the gentry feared what could happen if the populous continued to be allowed to own guns. Keep in mind, this happened after instances like Shay's Rebellion in New England and peasant insurgencies in places like Pennsylvania were very real threats to the gentry and their power. [Bouton, Terry. "A Road Closed: Rural Insurgency in Post-Independence Pennsylvania" The Journal of American History, Vol. 87, No. 3 (Dec., 2000), pp. 855-887). In both instances, militias were used to put down the protestors, and it is these militias (that answered to the local governments) whom were the only ones protected by this amendment initially. Supreme Court cases in the ensuing years would restructure this mindset entirely. So they created it in order to protect a State's right to arm and maintain militias if and when uprisings would come about.

This question is also very close to a question I had asked last week on gun ownership during the Revolution, so I am posting the link to that right here. I suggest reading that link since it will give you some detailed annalists related to your question.

Well, that implies that the right of the people to keep and bear arms was infringed prior to this amendment. Otherwise, there wouldn't need to be this amendment.

It doesn't imply that; while I see what your reasoning is, your reasoning is wrong. While the Bill of Rights was a set of changes to the Constitution, it was not added to fix actual problems that had already popped up with what the federal government was doing. During the ratification debates, some states were worried that the new constitution they were being asked to ratify would allow the newly powerful federal government to violate the rights of their citizens. In the ratifications, a number of states either said "we're ratifying this with the understanding that the Constitution does not allow the federal government to violate the following rights" or "we're ratifying this, but think it would be better with the following amendments and we'll push to add those through the process laid out in Article V."

Dealing with these was one of the first things done by the first Congress. At the beginning of May 1789, a grand total of two months after the First Congress was supposed to start (and just one month after the House of Representatives first established a quorum), they received an application from Virginia for a constitutional convention to propose amendments (Virginia's ratification had said "we think there should be changes, but that it's better to do those changes through the procedure in Article V rather than trying to get them made before ratification"). The application was dated November 1788, before Congress had ever met. The Bill of Rights was first introduced by Madison a month later.

It's a mistake, then, to think of the Bill of Rights as being designed to stop something objectionable that the government was already doing. It was designed to address some objections that states had to the Constitution as originally written. When he first introduced it, Madison admitted that part of the reason was to head off an Article V convention, which might result in more significant changes to the government and undermine the compromises made in the original convention.

/u/uncovered-history talks about some of the reasons why the right to keep and bear arms was included in that list of "things the government shouldn't be allowed to do." But the other part of your question is wondering why it'd be mentioned if it wasn't being infringed, and that's why: there was fear that without explicitly protecting it it might be infringed, and adding a Bill of Rights addressed that fear.