The first reason: it is very important that your website should be accessible through all modern mobile platforms with different screen sizes. The number of visitors viewing websites through mobile devices is growing and the latest data shows that mobile digital media time in is now significantly higher at 51% compared to desktop (which is 42%).
The second reason is Google 'Mobilegeddon' (somebody call it 'Mobilepocalyse' :). The idea behind this term is simple: Google will not rank your website high for mobile devices, if it is not mobile friendly. As a result the businesses lost Google search traffic (or get more traffic, if the website is mobile friendly).
So, if your competitor is responsive, and you are not, don't loose the time and start to plan your mobile strategy immediately! Otherwise your competitor will gather the mobile traffic and clients in your business niche. See more