Because more people use it (search for network effect).
I actually learned about network effects, when I did a paper on iPhone vs BlackBerry at my school.
One thing I remember is that network effects are strong only when a consumer has no reason to have two (or more) of the same items, like in case of a cell phone. Is there something about BTC that would prevent people from using other cryptocurrencies simultaneously?
Litecoin is a copy of Bitcoin with only a few changes and came out after Bitcoin, it doesn't offer any big advantage.
In theory Litecoins could be just as useful as Bitcoins, but there are only few reasons to migrate to them.
Not true. The dollar value right now makes me want to buy LTC more than BTC.