Either security agencies are really good at preventing them, or the assassins are really incompetent or maybe not interested (which seems improbable).
In Europe there used to be a tradition of politely capturing enemy nobles and ransoming them. They were quite upset at the reality of modern warfare where the leadership died just as easily as the common man.
I would imagine that the intelligence agencies frequently know where targets are, but only rarely carry out a strike.
He's tweeted about why he wanted to do it:
https://twitter.com/realDonaldTrump/status/38292347815791001...
https://twitter.com/realDonaldTrump/status/25578456090477363...
https://twitter.com/realDonaldTrump/status/14160455485582540...
This can be seen even in this case where Suleiman's deputy is considered more or less to be a clown even in Iran and the only reason he was promoted promptly is for the appearance of continuity.
Western democracies tend to be much less centralized in this manner, while killing off say the director of the CIA will have an impact it wouldn't be nearly as much of an impact as in this case.
The director of the CIA through oversight and a very different internal culture delegates much more, consults much more and in general relies on the support of others within the CIA and outside of it.
This is the natural outcome of living in a society that you don't have to fear that your underlings would work to seize power in a manner that would often leave you on the outs with the regime and your head on a spike.
This is because the transition of power is a common place in democracies, a CIA director knows that they are appointed by the administration and that their career is often limited to the lifespan of that administration. When it's over they know they'll retire to the private sector and could bill six figures for a talk or a lecture.
Sulieman knows that his retirement could quite likely end up being a 4 by 4 a cell for him and his entire family for the rest of their life and that the rest of their life might be quite short past that.
Beyond that both a successful and a failed assassination attempt would simply cost too much, especially if the country you are targeting is capable of waging conventional or economic war against you.
Iran for the most part isn't capable of anything, if it will try to close the straits of hormuz they will be opened by force with the likely outcome of the total destruction of the Iranian navy and airforce. They might be able to launch asymmetric warfare in the region but in the end their capabilities are also limited in that regard.
Hezbollah attacking Israel might cause some temporary financial damages to Israel but Israel can easily level lebanon. Attacks against US forces in Iraq are possible but this is nothing new it doesn't matter if these are Shia militias or AQAP... Attacks against Saudi Oil operations again would eventually be easily dealt with once they actually threaten the global economy.
The only real outcome I'm expecting out of this is some retaliation that rather than aim to inflict the most damage to a US or allied target in the region would be limited to something that Iran could use internally to say here we did something but not significant enough to provoke a major US response.
Basically the response for this has already be costed by the US government, they'll pull out non critical personnel, boost protection something would go boom that would result in more of damage to their image than to actual lives and then things would deescalate.
Because one thing nearly certain that if there would be anything close to the Marine Barracks attack in beirut or even to the USS Cole, there likely will be a direct and major retaliation against Iranian targets within Iran. And that is something that the Iranian regime cannot sustain currently, as it has too much internal opposition.
And whilst much of it's internal opposition doesn't exactly views the US as some sort of saviour they would definitely size the opportunity if the US weakens the IRGC enough. The Iranian military and the police would likely not get involved in a civil conflict, that's the reason the IRGC exists. In fact the Iranian military was intended to be dismantled but due to the Iran-Iraq war it had to be kept in place.
However in terms of allegiances the ayatollahs can only rely on the IRGC and the IRGC is quite disliked internally due to its corruption and control over the economy within Iran.