Your domain name should be easy to remember so that your customers or clients can easily find your website. SEO requires the same. In order for users to find your site through search, it needs to make sense.
A website with the domain name yourbusiness.com is likely to be afforded more authority by Google than yourbusiness.co.uk because users are more likely to type in yourbusiness.com than yourbusiness.co.uk or any other TDL for that matter.
As .com domains are the most powerful, they have become more popular. As with custom licence plates or the new fad food, this has led to an increase in competition for the best .com domains.