As a rates trader, your positions are denominated in the millions of dollars. Some back of the envelope math:
1mm of 10y treasuries (repo'd at EOD) is roughly $1k of risk (dv01 -- if rates move by 1bp, your pnl fluctuates by $1k). To finance that position, you're paying $236 at 9% vs $27 at 1%.
Now say you've got 1mm of 2y treasuries. That's roughly $0.2k dv01. Unless 2y rates move 1.25bp overnight, you're losing money just financing the position.
In short, there's a massive difference between paying 2.3bps and paying 0.27bps.