Google had PageRank, which gave them much better quality results (and they got users to stick with them by offering lots of free services (like gmail) that were better quality than existing paid services). The difference was night and day compared to the best other search engines at the time (WebCrawler was my goto, then sometimes AltaVista). The quality difference between "foundation" models is nil. Even the huge models they run in datacenters are hardly better than local models you can run on a machine 64gb+ ram (though faster of course). As Google grew it got better and better at giving you good results and fighting spam, while other search engines drowned in spam and were completely ruined by SEO.

PageRank, everything before PageRank was more like yellow pages than a search engine as we know it today. Google also had a patent on it, so it's not like other people could simply copy it.

Google was also way more minimal (and therefore faster on slow connections) and it raised enough money to operate without ads for years (while its competitors were filled with them).

Not really comparable to today, when you have 3-4 products which are pretty much identical, all operating under a huge loss.

The sheer amount of data and infrastructure Google has relative to their competitors.

Just having far more user search queries and click data gives them a huge advantage.

Google is in a two sided market. Their moat in search is their ads market share, their moat in ads is their search market share.