In a nutshell because of two reasons:
1) Energy from the sun is sustainable energy and is free. The cost of solar panels is going down each year while the technology becomes more advanced..
2) In the US, there are still great Federal and State Tax Exemptions and Rebates. Please read the article below thoroughly and take note that in 2019 the extremely lucrative 30% Federal Tax Exemption goes down and then in 2023 it’s gone for all residential customers. You still have time to get the full 30%!