Because Classic ASP is old and becoming antiquated. In my opinion PHP is just flat out better.
More than 80% of our customers were already on PHP and ASP and Coldfusion were falling drastically. We couldn't continue to justify the time it took to do things in 4 server languages at the cost of not being able to do things as quickly.
Our hope is that by focusing on the right server choice for our customers we can have an easier customer experience, enhanced feature sets, better QA and documentation, and do it all faster.