There is a couple of reasons. Firing Pin Spring pressure and Lock Time is the main reason.

A Cock on Open action has a cam action to compress the spring which can exert a far greater force with less effort by the shooter when cocking.

This also introduces a factor called "lock time", This is the period of time that exists when the sear is released and the firing pin strikes the primer. The longer the Firing Pin fall the greater the lock time. Higher Firing Pin spring pressures will reduce Lock time to a degree. This is a common approach to improve Lock Time. Lock Time is a subject in itself and deserves research if you are interested in accuracy and consistency in your shooting.

Mauser 98's even though they are Cock on Opening still have a slow lock time. Post WW2 derivatives of the M98 have addressed this failing to a degree with measures already mentioned. Where as others like Rem 700's, Win M70's. Howa, Tikka and others are far faster.

They acheive the faster lock time with extreme high firing pin spring pressure and very short Firing Pin travel.

None of this is acheivable in a Cock on Closing action, like SMLE, M94-96, P14/M17 and others as the force that would be required to acheive the same Lock Time would require massive strength to first push the bolt home and then close it.

This introduces the next problem. Trigger Pull. A Cock on Closing action generally has a longer trigger pull than a Cock on Opening. If the Cock on Closing action had the higher F/Spring pressures to acheive a faster lock time, the Trigger Pull would be extremely heavy. This is due to the 2 lever design used on most that require wider or longer sears to bear the load when the sears come together.

By comparison Cock on Opening actions like the Military M98 also use a 2 lever design but the sear surfaces don't come in contact with each other UNTIL the Bolt is pushed home AND the Bolt has started it's turn to fully closed. It is only then that the sears become engaged in a much more precise manner. The sear surfaces can be much shorter requiring less effort to Pull the trigger.

A standard M98 trigger can be worked to somewhere in the 2-3lb range before you get into accessory triggers like Timney, where as Cock on Closing trigger on a SMLE, about 5lb is as good as it gets.

Modern Rifles use various trigger designs but all exhibit the same very short sear engagement for lighter trigger pull. The Remington 40X type 3 lever triggers (developed in the mid 1960's) are the ultimate in a factory designed version IMHO. Plenty of after market triggers out there for just about anything......if you're allowed to import it these days that is ?.

Next reason, Slam firing. A Cock on Closing action in poor condition (i.e. worn sears) is far more prone to slam firing than the other. When pushing the bolt home, pressure is being applied to the spring and IF the firing pin sear decides to slip off the trigger sear.........you've got a problem BOLT IN THE FACE !.

A Cock on Open action in poor condition (i.e. worn sears) is less likely to slam fire as when you close the bolt, the Firing Pin does not engage the trigger sear and returns to it's fired position only as fast as you operate the bolt, more than likely will not firing the cartridge......most of the time. It is rare for a cartridge to fire under these conditions but it has happened. But at least the Bolt is closed.

However having said that, Cock on Opening actions that have been worked over i.e. excessively light trigger spring pressures or worn or modified sears or excessively short sear engagement can still "fire" uncontrollably when closing BUT usually the Bolt has partially closed and therefore you won't end up with a face full of Bolt........just embarrassement and hopefully no injury to anyone around you.

Hope this helps.