The reason is simple. The establishment has nothing to gain but everything to lose.

Let's say the maximum reach/exposure to voters is 100% in their country. The political establishment of many decades already have this number close to 100%, but the opposition may only have 5%. You see, the opposition has potential to gain a lot from ads, but not the establishment.

Also in a rich society like Singapore, grassroots can make huge contribution to the opposition party if their voice get heard and spread. But the establishment does not need this.