why does edgar lee masters title his poetry in spoon river anthology by names of different people??

This is a technique used by a lot of authors. The chapters are named after something or someone or a certain point in time that influenced them greatly. As you can see, the citizens in the book are TYPES of characters, not simply historical portraits, and this technique is used to show that the citizens are similar to famous historical legends that we all know and love(or in some cases hate), however the citizens are all a more general version of those famous people.

The reasoning for placing specific names for the chapter titles was for a commonly used technique of contrast, which stands out to many readers. The contrast being that the citizens portray historical characters through their general characteristics, however the chapter titles are very specific and zeroed in on a specific person, who may have influenced the major premise to that particular chapter.

I hope this helps :) If you need any more clarifying, don't hesitate to ask