Short version: Protective encourages you to sit back and develop during the early years, as well as pushing you to research towards Longbows and Crossbows. To do that you will need to go down the southern research path towards Feudalism and Machinery, which has opportunity costs that I talk about a bit more below.
In short I think it encourages the player to alter their playstyle and research priorities from some strong strategies (centered around melee units and Math-based techs) that have been developed and refined since the game came out. There might be some great plays to be made with a protective/religious game, but they seem to be more specialized plays which makes them less robust and flexibile, therefore harder to apply as general principles when playing the early game.
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When you compare Protective with aggressive its shortcomings are evident:
Unit Comparison: Early game, melee units are superior to archery units. Gunpowder units I'm willing to call a draw.
Cheap Building: Cheap barracks are leagues better than walls and castles. Barracks also scale and allow flexibility - it improves the value of each hammer you invest, and you choose which promo it goes towards. Walls and Castles are fixed investments that don't scale in value and are rarely used.
Promotions: Protective gets two free promos at no XP cost, and Drill was changed to open up the same promo paths that Combat 1 does. Protective has the edge here.
Looking at all of those, it seems clear to me that the cheap building is where protective falls apart. If Protective allowed cheap barracks I think it would often be considered superior to aggressive, but I've heard that it wasn't allowed to preserve the defensive flavor of the trait.
Or perhaps even something like a 10% universal bonus while defending inside your own cultural borders would be warranted. Or maybe remove the free CG promo but make it so that all archery/gunpowder units built in a city with walls get CG1 for free.
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The issue is larger than the comparison between Aggressive and Protective traits - it's also about which units and technologies it rewards you for pursuing. We've seen the dominant strategies in SP gravitate towards Bronze Working, Civil Service, and the cluster of techs around Mathematics (Calendar, Construction, Currency, Literature). It's rewarding to skip archery (put your beakers elsewhere) and Feudalism is prohibitively expensive to research in addition to being along a less robust research path (Religious instead of Mathematics).
Unit types: Archery units are strictly archers until the early AD years and Feudal Longbows.
Melee has more variety that you can tailor to fit your needs - robust and flexible, again. Spearmen, Axemen, and Swordsmen.
Machinery opens up both crossbows and macemen, so I'll call that a draw, but shortly after you can grab Engineering with increased road movement and pikes, which handily counter Knights, the beefiest unit on the southern/religious tech path (which again has lots of opportunity costs).
