Firstly, leasehold is a fully legal ownership form for foreigners, see Buying a villa. Secondly, leasehold makes a better investment compared to freehold property.

A good investment has the highest possible rental income for the lowest possible investment. This can be better achieved with leasehold property than with freehold property.

Freehold property in a top location with good rental income is very expensive and therefore the initial investment is high.

 For the same initial investment, it is better to buy leasehold property in a top location with excellent rental income than to buy freehold property in a less popular area with limited rental income.

With long term leasing in a top area the initial investment is relatively low but the rental income is very high. Therefore, the investment is returned relatively quickly.

The value of property in a popular area is steadily increasing and it is easy to sell the property again.