Short-run Phillips curve is upward sloping because:

If wages and prices were flexible, they would adjust quickly to changes in demand, eliminating the trade-off between inflation and unemployment. This would lead to a vertical Phillips curve in the long run, contradicting the upward sloping nature observed in the short run.

High competition typically leads to lower prices and higher efficiency, reducing the market power of firms to set prices above marginal costs. This competitive environment would not contribute to the upward slope of the Phillips curve, as it would promote flexibility in wages and prices rather than stickiness.

While aggregate demand (AD) is indeed downward sloping, this characteristic pertains to its relationship with the price level rather than the Phillips curve's inflation-unemployment trade-off. The shape of the AD curve does not explain why the Phillips curve is upward sloping in the short run, as it focuses on different economic interactions.