That's because the contracts are just too large. Although legislation has been enacted to end the practice, small businesses complain that bundling continues. This task becomes more and more difficult, however, when agencies take discrete, small contracts and roll them into massive procurements that are too large for small businesses to handle. Even when contracts are not bundled, small companies often have to jump through hoops just to place a bid. One hurdle is the extensive paperwork that's usually involved.
Most entrepreneurs have little time for that.
Ironically, another hurdle is the outreach effort that procurement officers make. Many requests for proposals require the officers to come up with a list of contractors who are qualified to complete the job. Since most small firms aren't even on the officers' radar screens, they're often left off. On the other hand, you may have a big business that they know well, that they do business with frequently. It does create work to encourage them to utilize some small businesses.