Because if you use WD40 to lubricate your BSA Mercury it will diesel like a farting elephant, ruin the spring and give inconsistent power and accuracy and have a short life. WD40 isn't a lubricating oil, it is a very volatile penetrating solvent for freeing rusted components and for dispersing water. After it has finished exploding in your compression tube, it will evaporate off and your gun will be running dry. You will need a new seal and a spring in only a few months.
Lots of people have used the Tuning Guide to make their guns better. Also, it is a tune that has been in use for the last 29 years so I think that it counts as 'the old fashioned way' at least as much as scrubbing with abrasive paper and WD40.
If you follow the instructions you will have a nice, smooth accurate rifle that will last a long time and will be a pleasure to shoot. Lubing with WD40 and changing the spring and seal will give a workable gun, but it won't give anywhere near the best performance it is capable of. If you HAD followed the guide then you would have smoothed out the cocking slot and you would not have knicked your seal.
Read it and try the guide, you might be pleasantly suprised!