Two big reasons. The first is that Magento 1 is at end of life - meaning it won't be supported going forward. We know - if it works now, won't it keep working? Well, yes and no. It MAY work for a while, but once something breaks - if you upgrade the server, or some extension, or something else - you'd have no resources for fixes.
The GOOD news is that Magento 2 is going to be supported for a long time, is faster, the backend is easier to use, and is has a ton more features to take advantage of.
Want to know more? Email or call us - we love talking about this stuff. We've done a ton of Magento conversions - from sites with 100 items, to sites with 100,000 items. We'd love to help.