The inertia of arms and equipment transactions is very strong. If their own R & D capabilities, economic strength is not enough, the beginning of the purchase of a country's weapons, the future maintenance, upgrading, are dependent. It is also difficult for the weapons of various countries to be compatible. South Korea, India's some self-developed universal brand patchwork goods just always have all kinds of problems.
Not only are parts from different countries incompatible when assembling a piece of equipment, but the same is true for the construction of the army's system. For example, military communications equipment used by various countries is coded differently. Then you can't interconnect the data of your multinational equipment.
And the equipment is definitely easier to adapt to subtypes of weapons from the same country when upgraded. For example, if you bought a T-72 before and now want to upgrade your tanks, then the T-90 is the first choice. Suddenly switching to the VT-4 would take the troops longer and more time and commitment to transition, learn how to use the new equipment, and how to command the new equipment into combat.
That's why most countries try to buy only the same equipment within a system.
In this way, those who have long dominated the arms market naturally have a great advantage. Unless the domestic equipment can do: 1, either compatible with foreign equipment (such as Type 69, a large number of exports to Iraq, Iran and other countries) 2, either cost-effective have greatly improved (such as Saudi Arabia bought the M1A2-S last summer by the Yemeni Type 69 frontal destruction. M1A2 foreign trade version of the removal of the poor oil armor module, and even composite armor and armor-piercing bullets is a simplified version of the Saudis to consider whether to change the VT4 try). Otherwise it's hard to take advantage of the market.
In addition there are some Zheng Zhi diplomatic reasons.