Is net neutrality not important in a competitive market of internet providers?
Yes, if there were more competition then net neutrality would not be such a big issue. Any throttling would cause an ISP to lose customers to competitors.

Competition is a main assumption behind why markets work. However, in this case an ISP has market power which breaks the assumption. The market power allows them to easily put smaller ISPs out of business. 

Here is a link to a rather long article on how ISPs get this market power. Essentially, the reason is that starting an ISP is entering an economy of scale, which requires a large amount of capital to start: http://arstechnica.com/business/2014/04/one-big-reason-we-lack-internet-competition-starting-an-isp-is-really-hard/