For the same reason most folks don't know what petition for redress means. They simply don't know they have the power to enlarge the number of parties, or to allow for 'open' primaries where the best individuals can be selected regardless of their party of record. Most resist out of fear of becoming a single party 'communist' society. (IE: a group of individuals with no apparent communl purpose all get elected and rule that the USA is now theirs to have forever, and void the constitution). Lack of knowledge leads to fear. Also, seeing how poorly patched together 4, 5, 10 party systems work or don't work, such as Israel, Brazil, and others that disband their Parliaments at thje drop of a hat, or other impasse. Some think Bicameral also means Bi-party. We have 4 MAJOR parties, two are very poorly represented, and the other two are represented poorly. (Few in numbers) and the two lesser known parties have never had anyone in the White House.