The financial consequences of orchestrating such an attack outweigh any potential gains that could be made.
That is not necessarily the case. There have been several 51% attacks against certain altcoins that resulted in financial gains by the attackers. Furthermore, these attacks did not result in the death of the coins.
Altcoins are seemingly subject to different rules. Bitcoin's participants are a mixed bag. We definitely have speculators, but we also have genuine users who require utility and security. Bitcoin's credibility is therefore a vital component of its value. Altcoins, for the most part, mainly just have speculators. All they need is hype and marketing to thrive because they have nothing of substance to offer. They can be attacked time and time again, because they never had any credibility to lose in the first place.