This is a question that's been asked with reasonable frequency, with several excellent answers in our FAQ, though there's always more room for more commentary on the subject. Examples include this and this by /u/elos, this by /u/elos and /u/DonaldFDraper and this by /u/eighthgear. To summarise the general thrust of these threads, linear tactics were adopted for three main reasons. Firstly, it made command and control of an army much easier on a battlefield choked by gunsmoke. Secondly, it allowed them to maximise firepower - tactics at this time were focused on putting out as many musket balls as possible. Finally, having troops in close proximity allowed them to better fight off cavalry charges or to carry out bayonet charges. Cannons, meanwhile, were slow, inaccurate, and easy to suppress with your own artillery or cavalry.