The prize money is based on the interest rate the Government pay on the the capital (the bonds everyone holds). Very recently that interest rate was slashed[1] hence the amount in the prize fund fell. The minimum "prize" is £25 so as a result of the interest rate fall there are fewer £25 wins.
Over the years there have also been changes to how the prize fund is split, possibly to give more £25 wins rather than for larger amounts.
[1] N&I recently slashed rates on all of their products and in most cases are not providing any return on investors money (0.01% interest rate). There are many reports that as a result there have been mass request from people to withdraw funds and N&I cannot cope with this demand - web site continually crashing and support phone lines not answered etc.