Many Interstates share numbering with US Routes. It's a way to keep around some US Routes that share some of their path with an Interstate but then diverge sharply in other areas without having to maintain two roads in parallel

Head over to Google Maps and zoom in on some random Interstates; you'll see the dual numbering. A good example is I-10 and Route 90 in southeast Texas. Everything between roughly Columbus and Vinton (including the whole Houston area), I-10 and Route 90 are the same road, but they go their separate ways on both sides. Or for a north-south example, I-35E and Route 77 in the Dallas area; they're joined between Denton and Waxahachie.

There's a simpler explanation. It's a pre-existing route that was adopted by the federal highway system. US 50 is well known enough that resigning it would be more trouble than it was worth.