There are some fairly obvious reasons from a military procurement viewpoint.
Due to necessity and through ingenuity, Israel is pretty skilled at manufacturing weapons.
Battlefield UAVs were pretty much invented by Israel, prior to that drones were mostly used for target simulations, though they have been some abandoned projects for reconnaissance/combat.
Israel pioneered re-purposing F15s as strike fighters, while their original intent was air superiority
Iron Dome is a practical and effective area anti-missile/artillery system.
These developments pretty much predated equivalent efforts by the top tier Western manufacturers like the USA, France or the UK
Though that it not always true, it seems like a lot of these systems are developed for relatively low costs. Israel just can't afford the pork barrel politics of F35s for example. Again, a question of necessity which makes them attractive to buyers.
The specifics of the SAM being sold
This seems to be a Barak. With a 100km range Baraks are mid/long range SAMs.
The Wiki link lists their equivalents:
- Tor missile system
- Khordad 15
- MIM-104 Patriot
- RIM-66 Standard
- RIM-67 Standard
- Aster (missile family)
- RIM-174 Standard ERAM
I recognize only some of those, like the RIMs, Patriots and Asters. Half of the rest are Iranian missiles, developed because Iran can't easily source weapons abroad - I'd rate those as "dubious". And some of the remainders are Indian-built.
Now, SAMs are easy weapons to "get wrong":
They operate in high threat environments, being pretty much the first thing any enemy air force tries to take out. They need to balance detecting enemies vs being detected by enemies - a SAM site is relatively hard to spot with its search radars turned off, but is also isn't much of a threat. So radars and electronics need to get that balance right and proof of that only comes from during actual operations.
Their acquisition systems can be spoofed, jammed or counteracted by stealth.
The missiles themselves need to track their targets and operate past active countermeasures like jamming or flares.
Actual war isn't always kind to SAMs. Patriots were applauded for shooting down SCUDS in Gulf War I, until later analysis seemed to indicate they didn't do much to usefully counteract what was essentially a souped-up V2.
Israel has been operating with near impunity in the Middle East, in places like Syria which has extensive Russia-supplied SAM systems (which kinda discounts at least the export variants of the Russian SAMs in the list above). US operations over Iraq in '91 & '03 show a similar outcome.
Basically, there are a lot of moving parts to a SAM. Unlike say a tank or a jet fighter, actual performance is hard to determine in advance. A nation with a proven track record with weapon systems has the advantage, even if the Barak has seen limited, if any, combat. And the extensive experience Israel has in avoiding enemy SAMs certainly gives them some insight to design better ones.
The Barak is listed as co-development India and Israel. It is common to have offsetting programs in weapon sales, where country A gets to build x% of a weapon system it buys from country B. But an India-Israel collaboration is more likely to have a significant role for India, as opposed to say India-USA where India will at best get a very junior role building rather than designing.
Also, it can't hurt (see K B's answer) that Israel and Pakistan don't have diplomatic relations at this point.
While India is a relatively uncontroversial customer for Western arms sales, this may be less the case with Azerbaijan, in the context of their ongoing squabbles with Armenia. Russia somewhat supports Armenia so they'd also somewhat drop out.
To achieve economies of scale, Israel is under pressure to support its domestic arms manufacture by exporting abroad and is, within limits, probably less exposed to domestic pressure groups against weapon exports than many Western countries.
Though there can be a lot of tension between Israel and its Middle Eastern neighbors, that hasn't stopped pragmatic dealings with some more remote Muslim-aligned countries. Relations with Turkey were a lot more cordial in the past and the Iran-Contra affair even had some Israeli involvement.
In short, for quality, price and relative ease of acquisition, Israel is a very credible and reasonable vendor for top-tier weapon systems.