I think what changed their mind was that the market changed. Mailchimp was run like a company that wanted to prevail in the market and not just grow and sell. That affected their strategy in a good way since it allowed them to spend time improving their services long term, but most importantly it prevented them from trying to grow aggressively for the sake of growing, which is a very very bad idea when it comes to sending bulk email.
In the end what changed is that e-mail based marketing was on the decline, with other mediums taking over. There's a point where an industry that stops growing fast eventually consolidates and it tends to get absorbed by groups that has other revenue streams.
