There are so many reasons. That's a very broad question. In short: Mismanagement, corruption, nepotism, incompetence.

Here's some links to past threads:

• "What's going on in Venezuela?" (7 months ago)
• This shitty jpeg wall of text is surprisingly accurate
• Consequences of the default (in spanish, tl;dr venezuela will be in debt forever)
• "What's going on economically?" (2 years ago, still relevant)