The reason for this is simple.
There are two major schools of thought regarding indexing the web.
One is to index everything and then decide what to keep. The advantage of this is that all metrics regarding sites and pages are available to make decisions.
The second is to selectively index pages making any decision before indexing the page. The disadvantage with this is that little can be known about a site or page before indexing. The only thing available are links that exist within the index itself. This is extremely limited.
Google indexes everything and will make decisions later as to whether to keep any page based upon the metrics it can develop.
Bing looks to links on sites within it's own index before indexing other pages.
The reason why this happens is because disk space and systems are expensive. Google has made the decision early on to index everything it can while MSN, now Bing, has decided that indexing everything is too expensive. As well, Google has engineered systems that allow indexing to be both cheaper and faster. For Bing, scaling, while possible, is not within it's business strategy. It is that simple. It is a business decision for Bing not to compete with Google because of scale and expense.
The reason for the difference is also simple. For Microsoft, search is not it's core business, just a part of it. For Google, search IS it's core business and therefore worth investing. Microsoft does not have the will to be all in where for Google, it is a good business strategy and has been the strategy from day one.
If you want more pages indexed by Bing, you simply need to have more links to more pages.