Operational security is really hard and requires constant dedication which most organizations can't keep up over time. Eventually the most professional organizations will slip up and make some of the mistakes you point out above. It's very likely that someone has spies or informants inside organizations such as Hizbollah and Hamas. But this can also have been a lapse in standard operations that was finally detected and exploited by Mossad after actively watching for a long time.
Hezbollah recently switched to using only pagers for communication, because they were worried about Israel hacking their phones. It's likely they all bought pagers at the same time because of that, because I doubt any of them would have owned pagers already
A ground breaking impressive/scary one. Being the enemy of Mossad was always dangerous, but it just got that much scarier.
How come they needed to replace their pagers AT ONCE recently? Has there been some great breakthrough in pager technology? I have a 5 year old phone and no desire to replace it for at least a couple years more.
How come everyone had the EXACT same model? Even if the Israelis didn't compromise it, even the most incompetent intelligence agency would notice somebody ordering thousands of an item with an usual order count of zero.
Anyone with a very basic knowledge of supply chain attacks knows you don't buy just one kind of item. If they bought fifty different kinds of pagers, this attack would've been impossible.
I'm sure even in benign cases, placing an order of thousands of items on some niche product causes a lead time of months/weeks. Never mind that the Israelis had to painstakingly modify every single one under the cloak of secrecy. Didn't that raise any flags?
And after they did order thousands of the same item, they didn't bother opening up even a single one?
Honestly I don't think incompetence could explain this, I'm 99% sure Hezbollah is compromised at a high level.