Why can't I get Steam Summer Sale trading cards by exploring my discovery queue?

Assuming that with "reviewing my shop list" you mean "exploring my discovery queue", you probably are not level 5 or higher. Check the FAQ:

How do I get Steam Summer Sale trading cards?

Steam Summer Sale trading cards are only available during the Steam Summer Sale. There are five ways to get the cards:

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Explore Your Discovery Queue

If you’re Steam Level 5 and above, you’ll get a Steam Summer Sale trading card once you view all the items in your daily Steam Discovery Queue. You can continue and go through your Discovery Queue two more times each day to get a maximum of three card drops each day. (If you reach Steam Level 5 before the end of the sale, you’ll be granted the cards you earned by exploring your Steam Discovery Queue during the sale).

There were similar constraints in earlier Steam sales, e.g. the Summer Sale 2015.

(There doesn't seem to be a site with the whole rules that is not personalized.)