The simple answer is that there are very few chances for such a feat. Pitchers are usually replaced when they're being ineffective, not when they're doing a good job.
I'd say you'd generally need have to have one of:
- Using an "opener"
- Injury to starting pitcher
- Hard pitch count limit
to create such an opportunity. Otherwise the manager is going to tend to leave the pitcher place to reward the execution and see if the starter can get the no-hitter alone.
The Angels combined no-hitter earlier in 2019: Taylor Cole started the game as an "opener" and would not be prepared to pitch a complete game.
The Astros combined no-hitter against the Yankees in 2003: started by Roy Oswalt, who had an injury in the first inning and had to leave the game.
At 6 innings and 92 pitches, Aaron Sanchez was pulled on a pitch count limitation. That's pretty low for a starter, but I couldn't immediately find the reason for the limit.
Use of an "opener" has been experimented with quite a bit recently and teams are more protective of pitchers with pitch counts. So you could reasonably expect the rate of combined no-hitters to go up over time.