From Twitter:
We were watching Starship Troopers this afternoon. We haven’t watched it in forever. Why would you send infantry to the bug planet instead of just nuking it?
The answer is pretty simple, although it won’t be found in the movie:
“Because there are times in which it is just as senseless to nova bomb a planet as it is to spank a baby with an ax.” — paraphrased from the novel
The Terrans were trying to force the surrender of the Bugs, to make peace. And failing that, a lot of the planets the Bugs occupied were ones humanity could colonize, and glassing each and every one of them would destroy a lot of prime galactic real estate.
And glassing Klendathu would leave a lot of Bug planets intact, who would then continue their war of genocide against Terrans.
Geek Gab 
In the movie, the bug nests were deep underground and hard to locate, hence “shoot a nuke in a bug hole.” Just glassing the surface from orbit wouldn’t accomplish much, assuming the bugs are resistant to radiation like many insects and other arthropods like scorpions.