Because BGP doesn't support ASN numbers larger than 16 bits; the 32-bit support is a pretty nasty hack, and for a long time, lots of equipment would not support it.
Running BGP with 4-byte ASN's is well supported by all major vendors by now. The issue is policies that use ASN:XXXXX communities which is 2-byte:2-byte.
There is an RFC to extend this to 4-byte for the ASN part but that is not widely supported yet.
https://community.cisco.com/t5/routing/how-to-set-up-in-the-...
https://datatracker.ietf.org/doc/rfc5668/
For small enterprises/end users (that might not define their own communities at all) this is usually not a problem but for larger networks or ISP's you end up with a compromise where you are truncating the 6-digit ASN or using a private ASN, neither of which are desirable. Arin let's you request a 2-byte ASN and if they have one in inventory (they usually do) you can get it.
32-bit ASNs have been a thing for close to 15 years now. Any older equipment likely wouldn't be able to handle a full routing table anyway.
I remember setting up BGP in 1998 for a local mega corp. They had 2 full T1s! I think there were only 50K routes.
IPv4 blocks are actually useful. If you run a datacenter it makes sense that you'd try to get something bigger than a /24. Why you'd care about having a 4 digit ASN I don't understand... easier to remember?