While XLM might be indeed significantly faster and cheaper, most of the exchange's trading pairs are in USD. When you transfer XLM, you need to exchange it for BTC to be able to trade with it, which is quite the opposite of what you're trying to achieve if you're trying to save on fees due to the exchange's trading fees.
Even if transaction fees have lowered recently, withdrawal fees from exchanges are usually higher for btc. I don't know why, but I find much cheaper to withdrawal amd transfer xrp than btc or eth, for example.
Maybe those coins that are more common among new users and investors have overpriced withdrawal fees from exchanges
I had a case on the Huobi exchange. For the exchange of coins that are not very well known to you, they considered a rather large amount in exchange for Bitcoin. It was more profitable for me to exchange them for Ripple and withdraw the riple. However, on the withdrawal from the stock exchange the commission was crazy.
Therefore, you should always consider that it is more profitable for you.