It's essentially a compromise. Searches are comparatively intense on resources, especially with bloated databases - as forums tend to have. On one hand, you've got a useful function but on the other hand having unlimited access can easily bring forums to their knees. The compromise is to stagger searches. This way you don't need to pay for a more powerful server while still having the functionality at the cost of a slight inconvenience. The limitation is also in place to protect the site from people with malicious intent - unlimited searches can really put a dent in operations.