There's also the ICANN factor. It takes years for ICANN to approve a new gTLD and in the time between the new gTLD(s) being proposed and the gTLD launching, the market conditions can change dramatically. As Bob mentioned above, the DotCOM bubble burst just before .BIZ launched and millions of domain names came back into the market. The same problem hit the new gTLDs. in 2005-2007, Domain Tasting was keeping much of each day's drop away from the public and domainers. The solution, as far as ICANN and some registrars were concerned, was more gTLDs. This turned out to be the 2012 round of new gTLDs and over 1,000 new gTLDs were launched. The problem was that large-scale Domain Tasting ended in 2009 and that artificial scarcity was gone. This lack of demand is one of the reasons that .BIZ most new gTLDs struggled to gain registrations.
The bursting of the DotCOM bubble affected the prospects .BIZ but it still has approximately 1.4 million registrations. Many of them are brand protection registrations and these are automatically renewed each year. Neustar tried discounting to build volume but it has to be done continually (only around 5% of discounted registrations renew in some gTLDs). Neustar's discounting was sporadic rather than continual. This means that the spikes in new registrations and the troughs from non renewals are more apparent. Over the course of Domain Tasting, the ccTLDs began to take off and this hit .BIZ badly because it was initially competing with .COM and . NET. The ccTLDs took the country level markets away from .BIZ and it has been losing market share in most countries. It also used to be considered a "must register" gTLD for brands (a brand would register in .COM/NET/ORG/BIZ) and that is fading now.
It isn't a failed gTLD but it cannot compete with .COM for domaining and the lack of marketing means that the public is largely unaware of .BIZ. Now that Godaddy has taken over Neustar's registry business, that might change and .BIZ might gain some prominence on the Godaddy websites. If that happens, and it is successful in driving awareness, then the prospects for the gTLD might be better but it will take about two years or so of continual good marketing.
Regards...jmcc