Greed.
At least in my opinion.
These people want bitcoin to fail precisely because they know it can work, but weren't invested in it in the beginning.
They want bitcoin to fail because they know it can work, and they want it to be replaced by something where they can invest in the beginning, and see explosive growth in value.
There will be alternative crypto-currencies that are successful, I don't think any of the current alt-coins are it but it will happen.
But as long as bitcoin remains around and dominant - none of them will ever see the kind of growth explosion bitcoin saw, and bitcoin will never see that kind of growth explosion again either.
I believe many if not most of these nay-sayers want bitcoin to fail because they realize this, they realize they missed the opportunity to make millions just sitting on their fat asses, and that opportunity will only come again if bitcoin does fail. Thus conscious or not, they write these articles hoping to influence a crash in bitcoin.
That's my opinion.
Put all the thoughts against bitcoin only in this category seems very arbitrary. Maybe there are such kind of people (and for sure that exist) but I think that not all are so. May exist some who don't know the real value of it (not only as a currency but as a product which has opened a new era because was invented based at peer to per). Then may be others who simply don't see it as useful as it is advertised by its followers. There are to many who don't like nor the internet (or use it rarely or only for few things like for example the use of email) and neither (even more less) to buy on it. For this kind of people bitcoin is worthless for true. And are at all not few in our days. Then may be others who believe on the words of Mike Hearn and see it as a failed product full of bugs which will bring its failure. I named only few other possible existing groups of people who see otherwise from OP bitcoin and according to their view do even their judgement about it. Which is different from that given from OP.