In Genesis and about a year later it seems that there were no transaction fees, only the block reward (50 BTC at that time).

I was looking for the first block that contained transaction fees, but it seems a bit difficult to find as there seems to be many without fees, and from time to time one would have fees.

You can see this by browsing blocks around the year 2011 for example: https://mempool.space/block/113639

What made some transaction have fees and others not have fees at that time?