Probably because of state or EPA emissions regulations; it might be possible to refit them with pollution control equipment like Richmond Pacific did with one of its switchers recently, but it might not be cost effective. It's also very likely that they just aren't needed anymore; railroads with excess motive power aren't exactly in the habit of keeping old locomotives from defunct builders when they've got locos that are newer or whose builders are still in business and for which spare or replacement parts are easier or cheaper to obtain. Also, #42 is the only one of the three that's operational, and the other two were used as parts sources for #42 and would take a fair amount of work to be made operational again (both #40 and #44 are missing brake valves, plus #40's traction motors have been removed, and finding replacement Westinghouse traction motors that are in good condition or are even rebuildable probably won't be easy).