I think you have mixed up shooting STOs and shooting regular ground troops.
STOs are MUCH easier to hit than regular ground troops cause they give away their position with active sensors and by shooting. The rules for targeting them are under the planetary terrain rules post: 100%/fortification*to hit modifier, which is 5x higher than what a non-specialized ground unit would have against them.
Regular ground troops, though, are 3x harder for bombardment to hit than for your ground troops: they literally use the same rules, but with a 2/3 penalty.
The result is that it's probably better to kill STOs from orbit than on the ground in terms of supply expenditure (though if the STos can kill your ships you are probably better off doing a dropship assault).
I originally thought that it would always end up cheaper to do a ground assault in terms of BP expended in supplies to kill, but that may not be true.
If you get all your Ground Supply as light vehicles, ground force supply costs 2.48 vendarite for 500 GSP. Infantry are cheaper, but I don't know the cost off the top of my head (I found the LV supply costs in screenshots on the original rules posts).
HCAP is 12? GSP for 6 shots, so 500 GSP gives us 250 shots. To get 5000 shots (so assuming 20% as accurate as the bombardment) we'd need 10000 GSP. This costs us 49.6 vendarite (and nothing else).
A 10cm, 30000km range railgun costs 14BP and so consumes 14/100/4 MSP per shot. 1 MSP is 0.25 BP. So 1000 railgun shots (which are EXTREMELY cheap as orbital weapons go) costs 2.5BP. This looks like a great deal for the bombardment! Except you had to sit at 30000km to kill the STOs and so you probably lost your entire navy.
A better weapon for killing STOs is a particle beam (since range doesn't affect orbit to surface accuracy, but DOES affect STO accuracy, shooting from farther away favors the attacker). Consider a Strength 6 beam (so plenty to demolish an STO if it hits) with 240,000km range and 15 second cycle time. This costs 94BP, and so consumes 94/100/4 BP per shot. At 1% accuracy, to kill 10 STOs, we'd spend 23.5BP. This is probably still cheaper than killing them with ground troops at equivalent accuracy, but consumes much more valuable minerals (gallicite and duranium instead of vendarite). If you use bigger, badder particle beams the cost goes up. The cost of using ground troops remains constant.
Of course, the missile you proposed to use instead cost 200 BP. And you'd need a lot more than one if there are any point defense STOs. So the ground forces kick its butt.