Sprint and T-Mobile are on incompatible networks (CDMA vs GSM) and the Nextel acquisition was, as the article puts it, "disastrous."

That doesn't play too big a part when both companies are still evaluating* their post-GSM/CDMA strategies. Both are also known for being behind the curve on this, as well.

* HSPA isn't post-GSM, and rumors around Sprint dumping the existing "4G" WiMAX, in favor of LTE, have been around the industry for years.

A bigger consideration is the spectrum licenses.