First, because if you parallel the existing resistor you can only go hotter than the stock value - which is already nearer the hotter end of the useful range - and you'll need to keep another resistor in series with the trimmer to stop the value getting anywhere near zero which would overload the tubes; and second, because when a trimmer is in parallel, it draws more current as its resistance goes down, which means you still need to limit it with another resistor or it will risk overheating the contact point as it approaches the lower end of its range, or at the least mean that the trimmer has to be rated for more power than necessary. So this is now three components, instead of two. You will need to replace the stock resistor with a larger value to solve any of these problems, but if you put the trimmer in series with a *smaller* value of new resistor, it's only two components and the current goes down as the trimmer resistance rises, so it dissipates less power and is self-limiting to an extent - simpler, cheaper and safer.
The stock value is 75 ohms. To get a useful bias range both above and below that, you would replace the stock resistor with a 68-ohm or 62-ohm (or maybe 56 or 47-ohm, depending on how hot you want to risk it!), in series with a 47-ohm trimmer. The trimmer will still need to be rated for about 2 or 3W, but these are obtainable.
