Let's tackle what is perhaps the most obvious reason - the fear of making a mistake. This fear can be crippling in its effect. One imagines the worse case scenario for each possible outcome and rather than move forward with a choice, it seems easier, or safer rather, to remain in the place of indecision. In that space, there is an assumption, albeit an incorrect one, that negative consequences only come with making a decision. This minimizes the feelings of torture that come with not being able to move forward with a choice. One of the main reasons this happens is due to a lack of clarity in purpose.
Why does this particular decision need to be made? What is the intent, objective or goal behind this decision? Being able to articulate the reasons for your decision, will pave a clearer path forward to making the right choice.
Clarity of purpose is the necessary first step one needs to take, in order to become effective in making decisions.