According to an email over at WPC Nokia’s customer service have replied to queries about WP7.8 on T-Mobiles 710 with the response that some carriers have decided not to push it out to their users:
"In response to your concern, we understand that you want to confirm if the Windows Phone 7.8 will be available for Nokia Lumia 710 from T-Mobile. We are sorry to inform you that there are a small number of operators that have chosen not to offer the update to their users.
It is important to recognize that this decision was made solely by the operator and these include the T-Mobile network in the United States."
Of course as they pointed out this isn’t Nokia’s fault, but Tmobile’s who probably decided that the carrier testing wasn’t worth the trouble. Still pretty disappointing however all is not lost seeing how there are ways to manually force the 7.8 update; which you can find by a simple google search.