What is the motivation for miners to include all recent transactions in a block?
Miners have a long-term investment in the health of the Bitcoin network. If Bitcoin collapses, then their expensive ASICs are worthless. Miners particularly need Bitcoins to remain valuable over the long term because their hardware produces Bitcoins over time.

If nobody includes transactions in blocks, then Bitcoins would be useless and therefore worthless. That would hurt the miners' long-term investment.

If this ever became a problem, transactions would just wind up with higher fees to encourage miners to include them. Right now, enough miners will include a transaction with a miniscule fee that there's no point in paying more.