(What follows is a layman's understanding; I have a physics degree but never studied particle physics to the kind of depth required to give an authoritative ans

(What follows is a layman's understanding; I have a physics degree but never studied particle physics to the kind of depth required to give an authoritative answer to this question.) Because any attempt to quantise gravity via the methods that worked for other forces leads to theories which aren’t any use to us. So far all such attempts have led to theories which are not renormalisable. Renormalisation is not an absolute requirement for a useful quantum theory; you can do perfectly good physics in a non renormalisable theory with a high energy cut off. But we only expect quantum gravity effects to be noticeable in high energy / curvature regimes so that's no good to us. We already have a perfectly good low energy theory of gravity that explains all observable phenomena after all: It’s called General Relativity. A non-renormalisable theory that results in infinities that cannot be eliminated in the energy regimes that you hope to probe is not a useful theory. The other source of difficulty is that existing quantum theories assume a flat spacetime background. Reworking them to account for curved spacetime makes the mathematics much harder, even if you only assume a fixed but curved background. So we need new ideas, because the existing ideas give infinite answers in the regime where existing theories fail, but even testing such new ideas mathematically is currently challenging & very few people have the mathematical background required.