Initially blockspace was theoretically unlimited, but practically kept at 32mb. The lower limit was later hesitantly introduced by Satoshi at advice from others because of the risk of a poison block "spam" attack that especially existed early on in Bitcoins history. That attack is now unlikely, but even if it was tried a limit at 32MB would be low enough.

The more recent (alleged, as there is no way to identify "intent") spam attacks on Bitcoin before the split and Bitcoin BTC after the split are actually made possible because of the limit being kept so low that it takes very little effort to clog the network. The higher the activity and the more users try to outbid eachother on fees, the easier it is spam it successfully.

Bitcoin Cash will probably move to an algorithm adjusted or other more directly market based block size in the future, in order to avoid central planning of the total amount of available blockspace.