In a bowl, whisk together soy sauce, olive oil, Worcestershire sauce, lemon juice, brown sugar, Dijon mustard, garlic powder, and pepper.
Place steak pieces in a large zip-top bag or shallow container. Pour marinade over the steak, seal, and refrigerate for at least 1 hour (or overnight for best flavor).
Preheat your grill to medium-high heat.
Thread marinated steak, peppers, onions, and zucchini onto skewers, alternating ingredients.
Grill kabobs for 8–10 minutes, turning occasionally, until steak reaches your preferred doneness.
Remove from grill and let rest for a few minutes before serving.