One factor is that people's enthusiasm for Bitcoin drives them to step well outside their comfort zones and do projects they wouldn't otherwise. Bitcoin is a new startup economy and the plethora of opportunities is tempting. But the lack of experience or skill can lead them to mistakes which bring about the end of the project. Bitomat.pl is a great example - the owner tried to make use of AWS without having a clear idea how they work.
Another factor - especially with business ventures - is overestimation of the Bitcoin market size. The market is very small, and I have the feeling that people's general excitement about Bitcoin isn't matched by their willingness to actually use it. If it's a niche service the target market is the intersection of people in this niche with people who are using Bitcoin, which is tiny, and for more mainstream services, those that are founded just to offer the service in Bitcoin are often inferior to established mainstream offerings (see above point). It comes down to the revenue not justifying the overhead costs, so the project is abandoned.
Creating a successful alt currency is an immense undertaking, and so far has mostly been tried by people who seem to think that changing a few parameters is all it takes.
I don't have any specific suggestions how to avoid this, other than trying to be realistic about the viability of the project and the personal ability to carry it through. This shouldn't, of course, come at the expense of being a bit bold and daring.