Sex work differs from most other type of work in one very significant way - it's an industry in which capitalists cannot really control the means of production unless slavery (ie, a person can become the private property of another) is legalized and institutionalized. In other words, a sex worker - for the most part - is not as easily coerced into selling their labor to capitalists like most workers can be, and capitalists hate when people have a way to opt out of being hosts for their parasitism.
Sex work also has a way of subverting patriarchal norms upon which the status quo rests.
This is not to say that sex work is automatically a revolutionary, anti-capitalist or even "empowering" thing by itself - there are plenty of ways in which our socio-economic systems allows and enables de facto slavery without calling it slavery - but it certainly doesn't fit into the neat class hierarchy that capitalists wants society to be trapped within.