Because everyday day police waste hours of time that could be spent tackling real crimes on petty cannabis related crimes. Not only that there is thousands of pounds to be made in the taxing of cannabis rather than letting the criminals make all the money. The only negative points about cannabis in this day & age are the toxins & chemicals being used by the people who sell it to increase the weight of their product, sometimes using fibre glass or actual glass, however if the goverment were to legalize it they could control the strength of the strains that are available and make sure all the cannabis in the country is clean.
The main point I am trying to make is that the governments problems with cannabis seem to have no just foundations what so ever as the only problems are the people who are selling it and that could be sorted simply by legalizing possesion and setting up government controlled growing facilities, these would quite easily pay for themselves using the money made from the sale of cannabis.
You've got yourself into this financial mess, you're looking for money from anywhere cutting the most ridiculous things. Why not make some money off another taxable drug, you have no problems making ridiculous amounts of money off taxing cigarettes which are litterally death in a packet.