The main reason why poker isn't taxed is generally in UK taxation also has allowances for losses. i.e if your business is loss making you can offset it against other income or future profits (or even carry it back in a business start up). Now most gamblers lose including in poker. If they taxed poker income they would have to alllow poker losses. Used to have this arguement often when in the Revenue as a lot of my client base were race horse owners in North Notts/ South Yorks.
Should it be taxed? I do have some sympathy with the view that all should contribute to society (If they can). I imagine the most a poker player contributes is tax on ciggies and booze

. It would just be a real bugger to collect/assess.