Initial drizzle in the second sector during the race's second half bloomed into heavier rain, which prompted a series of cars to make the leap onto the intermediate tyre in a bid to anticipate worsening conditions.
Valtteri Bottas and Lance Stroll led the switch, with Zhou Guanyu and Alex Albon following suit.
Alonso subsequently made his first trip to the pitlane at the end of the 54th lap of 78 and, against the run of play, collected a set of medium tyres.
But the rain had worsened considerably by his outlap, and he quickly conceded defeat and followed Max Verstappen into the pits for intermediates.
This added stop put Alonso over 20 seconds behind, costing him a realistic chance of breaking Red Bull's stranglehold over victories in 2023.
Alonso was surprised that the stop had come into question, and reckoned that the circuit was mostly dry enough for slicks at the time of the pit call, having only anticipated a small shower at that point.
"I heard this question in the TV pen as well, which I was surprised about," Alonso explained.
"I didn't live the race from the cockpit as probably you did on the outside. For me was very clear that the track on that lap we stopped was completely dry, apart from Turn 7 and 8.
"So why will we put on the inters, when it was completely dry, 99% of the track?
"I stopped for dries, the weather forecast was a small shower, and the small quantity of rain as well was what we had [on the radar] as a team.
"We had a lot of margin behind us to put on the dry tyres and if necessary, the inter tyres. Maybe it was extra safe, I don't know.
"That minute and a half that it took to go through Turns 5, 6, 7, and 8, it changed completely. So the overlap on the dry tires, it was very wet when I got to those corners. But the lap that we stopped, it was completely dry."
Alonso explained that in dry conditions, he did not expect to have a chance of beating Verstappen to victory, and commended his team for at least being "aggressive" with its strategy to go opposite to the Red Bull driver on their starting tyres - Alonso opening the race on hards relative to Verstappen's mediums.
The strategy had been employed on the hope that the medium tyres would degrade more than the hards, but Alonso was surprised by the low degradation of the yellow-walled compound.
"I think we didn't have a chance, to be honest. I think we were brave on the strategy; it's not normal that you start on the first row and you choose the hard tyre, trying to do the opposite of the leaders.
"That shows, the commitment from the team and the how aggressive everyone was in Aston Martin to try to get the win.
"We knew that it was some downside to that strategy. We could ended up maybe P5, P6, or something like that - out of the podium.
"But this morning we discussed and we said we had couple of podiums this year, so we go for all or nothing. I think the strategy was good. But the medium tyre was behaving surprisingly good in our opinion.
"We were hoping for more graining or bigger degradation, and Max was able to drive 50 laps on an amazing pace.
"That was the reason why he won the race: not because of the strategy, he was just faster than us."
