What they said, and an increase in a number of factors:
- Better sensors.
- Better computers.
- Better software (AI/ML).
- Nationalism & economics
- National pride is a factor for countries with the resources.
- Local economic development is driven by talent.
- Fun projects retain/attract talent.
- Development of civil and military capacity.
- International cooperation
- At least some of the newest sensors augment those sent by other nations.
- Multi-national funding allows more probes.
- Getting the funding requires spreading the development around a bit.
The Hope, Emirates Mars Mission is a good example that encompasses all of the above points, that also goes beyond nationalism and economics, with it's Arabic name al-Amal, intended to send a "message of optimism to millions of young Arabs".
The projects don't have the same time scales. They didn't all start in the same year, and the small launch windows, spaced at just over two years apart, all combines to make the odds higher, that in any given window, there will likely be one or more probes heading towards mars. In my life-time, most of those windows were not used. Barring a world war or other world economy crippling disaster, over the next few decades, the rate will likely increase.