What limits the use of "solid state variable capacitors" like these for dynamically filtering (low power/received) RF signals?
The main issue is bandwidth.
Although there are some varicaps that claim a 10:1 variation (or more) in capacitance, not all of this is useable. One end of the range may be low Q, or have a very steep curve. Once you have a varicap on a board, with pads and bias connections, you'll often find that the useable capacitance swing at the tuned circuit is no more than 4:1, giving you only 2:1 swing in centre frequency.
Even if you did have a varactor that gave you more swing, or a construction technique that reduced the effect of strays, changing the LC ratio alters the impedance of the filter. There's only so much change you can make to the impedance of the resonators before a good filter becomes a bad filter.
You may only want to operate in an octave. However you've asked specifically about SDRs, and the main benefit of those is a very wide frequency range.
You could switch inductors into the filter, with the switching strays further reducing the capacitance swing available. Or you could build multiple filters and switch between them. Both of these add to the complexity. Once you have accepted the complexity of further switching, it often makes more sense to build a few, good, fixed frequency filters with stable components and switch between them.