I asked the question about why don’t we do this about 15 years ago on this forum.
Nobody knew why.
The reason now is due to the massive amount of money the DVLA make from the online sales and auctions, combined with the fact that unlike other countries where vanity plates are very often linked to vehicle taxation, so can 'die' after a period of time;
the DVLA cannot re-isssue a number plate which has previously been issued, so then a rarity factor comes into play for 'good' initials.
Supply and demand much in the same way that one 3 x 1 plate could be up for sale for a 5th of the price of another 3 x 1 plate.
If more people prefer letters first over numbers first, that would create more demand for them pushing up values.
I prefer letters first personally.
Might be because I arrived on the planet when letters typically came first on suffix registrations.
Who knows!
A 3 x 1 plate value value is to some extent determined by its age - but much more importantly the % commonality of the letters when cross referenced to how often initials occur in peoples names.
Ie: YUV 22 is worth far less than AEC 22, or DRH 22 - there are even better combinations than those 2 examples, I have a chart with all the first, second and surnames and cross reference that to determine how common each letter is and thus a plate potential value.
Clearly that ignores the name factor, like AMG, JAG, etc, etc.
Obviously 2 x 2 or more interestingly 2 x 1 plates follow a similar pattern, and reversals are worth less because they are newer, so more likely to have 'survived'.
My first and last initials were first issued in 1917, but were issued in reversed format in 1961, so obviously my 1917 plate has a higher value and (imo) looks much nicer than it's 1961 reversal; fwiw - both have 'survived'.