Mega Churches can likely be explained in part by the fact that more modern denominations, including Baptist (Even non-denominational) have integrated together with each other a lot. People are starting to realize that on a basic level, everybody is teaching the same thing; the love of Christ and desire to be like/with him.
I've asked several people (most not even from my church) and 90 percent admitted to being non-denominational or passive about where they go on Sunday as long as there is a good christian influence and a love of God. This means that in an example town, there will be one huge church and a handfull of smaller ones, rather than tons of same-size-small churches.
Of course this can't completely explain the phenomenon.
Mass urbanization, along with greater ease when it comes to getting places, as you said in your question, is a very likely factor into why churches of such massive sizes have sprung up. My church started in around 2000-2003 and has several thousand attending every weekend!
I'll bet that if you were to make a map of all of the 'Mega Churches' and overlay it on a political map, the churches would line up pretty closely with cities of population from 400k to around 700k.
Acceptance of 'Non-Believers';
Along with the increase of population at churches came the popularization of 'Christian Contemporary' music. It appeals to people who may not necessarily be christian because of its good vibes and heavy beat more closely matching that of 'Normal" music. I know several people who listen to christian pop and admit to not even believing in Jesus Christ.
Churches are also making a greater effort to have effect on their personal community, not just nations across the globe. This encourages those attending the church to reach out to their neighbors though increased fellowship with them.
My pastor said this sunday
America has never been more spiritually minded as today.
Then he said something about it not being christian spirituality necessarily. I've forgotten his exact words.
My point being that the idea of religion has grown in popularity, and simply, christian churches are everywhere, and open for folks to just walk in and here the message. There are a variety of different styles of churches, and people who don't like one style, may like another, so they end up joining a church rather than no church.
I don't claim to know what is happening with these Mega Churches all the way, maybe God has some really big plan for it.