Treehugger has an excellent article on issues with front loading washing machine odors. It’s a well-known issue, not to most consumers but to industry experts. There is some debate on the causes, but the main ones appear to be:

1.  US consumers not used to using front load washers. For example, one of the ways to reduce risk is to leave the door open after use.

1.  The manufacturers themselves not having much experience making front loaders. The biggest issues are with older models of front loading washers, which clearly shows that it was at least partially a design issue.
2. Consumers using too much detergent. This can lead to detergent buildups, and mold/mildew are attracted to modern, more gentle detergents.
3. Low water temperatures.
4. Fabric softener. It leaves a film that becomes a food source for bacteria.

There is a company that is doing a thriving business selling products which solve the smelly washer problem, fittingly called Smelly Washer. They are succeeding, because many times these odors are not removed with standard cleaning products such as bleach. Another company called Washer Fan makes a fan you can attach to the back of the front loader, so that it provides a 24/7 flow of fresh air to the inside of the machine.