Crowding in the Bronx is comparable with other parts of the system. The Lex in Manhattan isn't. For a Bronx extension to do any good without a station at 125th/Lex, you'd need to have connections to at least the and probably the as well. That's 2 more stations, minimum, plus an underwater tunnel. We're probably talking 2-3 times the total cost for the phase. A station at 125th would also make it easy to construct a crosstown extension to connect to the , which would greatly improve mobility and allow customers to get around issues.
You see, the idea behind SAS isn't only to get people on the Upper East Side to take the new line. The idea, Phase 2 and beyond, is to get people going to destinations along the (and eventually the ) to transfer at 125th Street, north of the most-crowded section of the Lex. If there is a station at 2nd and 125th, there won't be an easy transfer to the Lex, so Bronx traffic wouldn't use SAS.