Why are the proletariat a "genuinely revolutionary class"?

The proletariat are the roots of revolutions throughout modern history and even before that because they are the most routinely exploited and abused peoples of society: slaves, factory workers, field workers, miners - these are professions we think of as dangerous, back breaking and underpaid.

These are the classes of people with the least to lose in a revolt and the most to gain.  It is also almost always the largest class in terms of numbers, and it is through numbers, Marx argued, that the proletariat can find unity, and with unity it can find strength.  Strength enough for a revolution that is, and in many cases they were successful.