as sheldon sort of said, spokes have way more stiffness along their axis than they do perpendicular to it. i mean, pull on a spoke. does it deform (stretch)? now push it sideways. sure, they're stiffer under tension...so push a spoke on a built wheel sideways. it still moves a hell of a lot more than it does axially.

that's why radial spoking in the rear is a bad idea.

up front, the primary stresses on the wheel are in line with the spokes. your weight bearing down on the hub, pushing on the spoke(s) at the bottom of the wheel and pulling on the one(s) at the top. even cornering or rocking the bike in a sprint the lateral loads on the rim are still in line with the spokes as they are stretched or compressed. no problem for a radial build.

the same is true in the rear, but there is an additional force...you pedaling and pulling on the chain, trying to spin the hub independently from the rim/tire, which is stuck to the ground. it's a job for the spokes to transfer this force to the rim. a radial lace has the hub pushing on them sideways, just resulting in flex. whereas a crossed lacing places the spokes more or less tangent to the hub flange, aligning the hub's rotational torque with the axis of the spoke. no flex.