Fossil fuels and renewables are just categories. If you take away those categories you'll see that the growth in solar, wind and other sources that are classed as "renewables" are not significantly outpacing the growth of natural gas. The reason this is significant is because the growth of natural gas is as a result of those who see it as a long term investment. The new natural gas power plants/generators were almost certainly built to return a multi-year profit to their investors. Don't let the drop in coal distract you from the growth in natural gas, it represents a long term shift in the energy mix of the US, and will be almost certainly harder to shift than the coal industry.

What you should be asking is, why are individuals investing more in natural gas rather than renewables? If we were being honest with ourselves, the problem is still battery technology. Investment in electricity storage is costly, and the batteries we have today become less effective the more they get used, resulting in regular replacements being required to maintain storage capacity. There are groups working on the grid storage problem, but it's far from resolved. Without it being resolved, there's a ceiling beyond which solar and wind are not likely to grow, as the most important factor in grid electricity is reliability, and storage is the only way to make a grid with the majority of energy coming from wind and solar to be reliable.