Same as version 1, it makes it simpler. Version 1 addresses are just a specific script, while pay to script hash addresses can be anything possible with bitcoin. SegWit is also just a specific script where the signature is elsewhere.
I see, so the version 3, will just phase out slowly the addresses, but I hope support for v1 will remain forever, in the protocol and in electrum, since most people store their money in cold storage.
Folks who have 5000+
BTC would probably not like to change their addresses often, so at least 20-30 year support for v1 would be good.