I live in the US, so I'll answer this question as an US citizen. Yes, while it is true that US citizens have a right to bear arms via the US Constitution/Bill of Rights (assuming you qualify and aren't a prohibited person - e.g. Felon, DV convict, mentally adjudicated defective, all other disqualifying factors, etc.), each state is different. Some states such as NY, CA, WA, IL, OR, CT, MA to name a few, are really stringent on firearm ownership and may have stricter guidelines especially CA and NY. So just legal ownership and acquisition alone would be a high barrier of entry to access them.
Then after acquisition and ownership, there are other factors too. Some people would prefer a more peaceful way to CTB, like inert gases, N, or other substance. Personal preferences as well such as open casket (funeral), sentimental value, and more. Finally, don't forget that even with a reliable method, the damned SI still gets in the way.
These are just some common and major reasons why not all US citizens even those who can legally and easily acquire firearms don't use said method. Personally, I'm going with the firearm method myself and it's because of it's reliability and accessibility for me compared to other methods.