Are you using Bing Webmaster? If not Try it to check your site's status with Bing : http://www.bing.com/webmaster
The Reason is that google crawl through websites more often than bing. Therefore you will find that google will index more pages than bing. I wouldn't worry about it to much.
Bing is not google..! They cant track us like google can.. i mean they take time to index our sites..!
Have had the same problem with Bing, their quick at picking up the homepage but deathly slow at picking up the rest. This even seems the case when your ranking for target keywords in their serps.
aaah.... u r another victim of bing i think haha I don't like Bing personally, coz its slower than google. I remember, I had to wait for more than 2 months to be indexed in Bing.