There are two reasons.
(1) First is simpler, end-to-end connectivity. If both source and destination have public IPv4 (or IPv6, of course) address, they can connect to each other in any direction anytime.
Your IoT with private IP
192.168.0.52 however can use NAT ONLY to connect to any public IP on the Internet whenever it wants, but the rest of the Internet cannot connect to it. There were kludges like DNAT and uPNP that used to allow you to specify that some incoming connections are enabled, but they are breaking more and more nowadays due to implementation of CGNAT because of IPv4 shortages.
A common (so-called) "solution" to this problem is that all your (NATed) devices connect to some central location with public IP (usually hosted by the manufacturer of device). This makes it work technically, but involves a privacy issue (you're giving all the data from your IoTs), security issue (as you're wide open to them, breach or disgruntled employee can do anything your IoT device can do and access), and reliability issue (when the manufacturer goes out of business or decides to stop supporting old devices or is suffering outages) all your (and everybody elses) perfectly functional devices will stop working.
(2) second problem is that it will stop working anyway (even for outgoing connections) some time in the future (not in a year or two, but still. The more IoT and services catch on, the sooner it will start breaking).
That is because NAT allows private addresses like
192.168.0.52 to reach the Internet at large. It does that by changing source address
192.168.0.52 to public IP of your router, but replaces source port with free one from the pool.
For example, your first connection might be
192.168.0.52:1000 might be (CG)-NATed to (public IP)
198.51.100.1:1000, and your neighbour
192.168.0.77:1000 might get NATed to
198.51.100.1:1001. Your second connection from
192.168.0.52:1001 would then be NATed to
Problem is, even simple stuff like opening a web page will likely open dozens of connections and use a dozen of ports (for DNS queries, HTTP(S) connection for different elements, JS analytics on different sites etc).
More expensive programs, like torrent clients, will easily use up a thousands of ports. And there is only 65535 ports available for any IP.
Which means several of your neighbours sharing the same CGNAT IP use a bigger share of connections (and more IoTs will mean more connections), and suddenly all of 65535 ports on that public IP 198.51.100.1 are used. Which means no new connections can be established for you and your neighbours. Which on bigger scale means lots people are cut from their IoTs, and civilisation as we know it collapses :-)
Since we would like to delay this civilisation collapse as long as possible, we're transitioning to IPv6 instead. Please support continued existence of this civilisation by using IPv6 if possible. Thanks!