Any idea why won't my car start? Just purchased new battery two days ago.

Double check the connection on the battery terminal posts. They may not be making a good enough connection. I recommend removing them both and then clean each one with a wire brush, go ahead and do this with the battery posts as well. This will remove any corrosion that may be built up and also this will scuff the cars battery cable terminals. Wipe off any metal dust and then apply battery terminal grease, you can purchase some for a dollar or two at any auto parts store. And last but not least, make sure you double check polarity on the cable terminals before putting them back on the battery posts. Then tighten them nice and snug but be careful not to over tighten as this could break the terminals