Few things from the top of my head:
- If we're talking about utility token, then you might want to guarantee investors, that for 5 tokens they can buy your product, with XLM you can't promise that because of rate fluctuations
- You might want a way to identify who your investors are, for example to pay them dividends or to give access to something or simply a discount for your services.
- ICO attracts users by promising return on their investment. Token price usually correlates directly with how good or bad project is doing. So good performing ICO token may grow 2x-50x and that's what investors are looking for. If you just give them XLM, then it's performance is much more complex and depends on how much SDF decides to liquidate today, you may not see correlation with project success at all.
