In addition to the technical factors mentioned by MCCCS, it's important to consider economical factors as well.
If the problem being solved has value outside of the Bitcoin network, it allows miners to essentially "double dip" on the rewards - they earn both from the Bitcoin received as block rewards, as well as the incentive structure off chain.
For example, if the problem being solved is character recognition, it would enable a number of companies who seek good character parsing systems, such as Google, self driving car companies, etc, to double dip. These companies are then incentivized to build a a mining farm to not only train their programs, but also use that training to earn Bitcoin, subsidizing the operation they're already doing.
Being such large companies, their primary goal is likely to have a good text recognition system - the value of Bitcoin earned is secondary. This allows them to invest much more into their mining farms, potentially outstripping other miners who do not have a dual incentive. Moreover, as they get paid twice, it is in their favour to attempt to resolve minor reorgs to build on top of their blocks - they are already subsidized via the off chain incentive, so the cost of losing a block proportional to their operating cost is much lower, allowing them to take riskier routes to resolving minor forks.
This also reduces the cost of mounting attacks - if the mining power required to launch an attack is subsidized by external incentives, it is easier to pay for and acquire.
This also changes the reward structure overall - if the majority of the earnings for a miner are from an outside source, they are less concerned with the effects of bad blocks, 51% attacks, or other bitcoin issues that drop the value of bitcoin. The risk:reward calculation for attacking the chain becomes skewed, allowing for high reward attacks to have a lower cost than in a single use system.