Why does this PUT oscillator max out at about 25 Hz?
The PUT is a nonlinear device that "switches on" when its anode terminal reaches a voltage determined by the gate terminal (approximately one diode drop above). It "switches off" when the anode current drops below a certain threshold known as the "valley current" (set in part by the source resistance of the gate terminal).

When your potentiometer gets down to 52K or so, it is capable of supplying enough current to keep the PUT turned on. At values higher than this, the current is low enough to allow the PUT to switch off.

Curiously, a "programmable unijunction transistor" as shown in this datasheet is really a 4-layer/3-junction device more akin to an SCR than an actual unijunction transistor.