2. It's harder to avoid the fuzz. Police techniques have gotten dramatically better. Everything from witness protection programs (Now you can testify against the mob boss without worrying about feeding the fish) to analysis of crime patterns means that police are collecting more evidence than ever before.
3. Entire avenues of crime have disappeared due to 1 and 2. For example, auto theft has declined dramatically due to the institution of chipped keys, GPS tracking, and the easy apprehension of chop shops.
4. While violent crime has declined in profitability, white-collar crime has become much more lucrative. There's big money in identity theft, for example. This means that the ambitious criminals who would have previously been robbing banks and mugging people are now doing their crime online and over the phone.
I don't think that these things are exclusively responsible; I'd just like to add them to the nice list that you have going.