Property with no other successor has to go somewhere.
The amount of property at issue is modest. It is about $7.5 million USD a year (in an economy with a GDP of $3,131,000 million USD a year), which is about 9 pence per person per year, and the King has pledged to donate all of it to charity anyway.
Anyone who really cares about where these assets go can prevent the escheat of their assets to the King by writing a will (which need not be very expensive).
Indeed, the threat of that possibility probably generates significant income as a marketing bullet point for solicitors trying to encourage people to draft wills (see, e.g., here), which probably creates low key political opposition to change from the fairly influential barristers and solicitors lobby.
In the case of monarchists, especially Tories who are members of the historically most pro-monarchy political party, reforming this practice might be perceived as anti-monarchist. This is because a change would directly harm the King's personal financial situation (should he cease to donate it to charity in the future), and because it would indicate a lack of trust in how the King choses charities to benefit from it. But opposition to the monarchy is an impression that the Conservative Party would like to avoid.
And, people "who pass away without leaving a will or next of kin" are not know to be a powerful political force.
All of this adds up to little political enthusiasm for changing the status quo.
Also, reopening the issue could lead to intense fights over who should get the money instead. This would invite unnecessary conflicts between interests that are otherwise not at odds with each other. Few politicians are interested in creating political problems for themselves of this kind that they didn't have before they poked this hornet's nest of a political issue.
Not to be entirely tongue in cheek, but before the practice of bona vacantia was established, in Bronze Age and Iron Age Britain, "taking it with you" by having many of your significant possessions literally buried with you in your grave as "grave goods" was as a common practice that solved the problem in a different way than we do now. And, I don't see a lot of agitation for a return to that practice.