I think that the lower price after the split is the reason. For one, the lower price will make stock seem to be cheaper. And another factor is that people are maybe prepared to bid , say 10 cents above the current price (on a limit order). Then that 10 cents is a greater percentage of the stock after the split than before the split.
Compare this with volatility, stocks priced below $10 are usually more volatile than stocks priced above, say, $100. (Note: just judging by my own observation, not a scientific research)
Best,
Rien.