Spanish and British coins were prevalent in circulation - the US mint only made coins if they were necessary. There was great demand for half dollars from banks as they made transactions easy. Remember: Gold coins at the time were worth more than face value, so they were struck as bullion, hoarded, and melted down, resulting in their extremely high prices now. Half dimes, dimes, and quarters were not as necessary, and there was little demand, thus fewer were made.
If you really look at it from 1807-1830 other than bust halves which were the biggest silver coin issued post 1804 there really wasn’t a lot of silver coinage struck period. No half dimes until 1829 and sporadic strikings of quarters and dimes and no particular huge mintages which most circulated heavily. A lot of ag-vg survivors today a lot of Spanish silver was in circulation until 1857 and assorted other foreign coin there was a great shortage of hard currency in the us until the industrial revolution. Much trade and barter more than cash money
The 2008 bust quarter book by Steve Tompkins notes that in the 1810s and 1820s, Spain was losing control of its colonies in the Western Hemisphere, which would’ve affected the supply of new Spanish coinage migrating into the United States. Presumably this contributed to the number of bullion depositors that wanted quarters from 1815 and onward. That number still pales in comparison to the number who wanted half dollars, though.
Most of the Spanish silver coming into the US prior to the end of the Napoleonic Wars was not coming from the Iberian peninsula but from the mints and trade with the Spanish Colonies in the New World. That trade actually increased following the independence of these colonies who simply revamped the appearance, but not the weight and fineness, of the coinage. Dollars from Mexico City after independence (via the Santa Fe Trail trade) and the other Latin American mints were considered to be as much legal tender as those coins from the same mints when they were under Spanish control, as long as the new republics kept the silver content the same as under Spanish control.
And it isn't until the coinage act of 1857 that Spanish (and other) coin is no longer legal tender in the US. Up until then, they were legal tender although the valuations changed. For example 1806: http://memory.loc.gov/cgi-bin/ampage?collId=llsl&fileName=002/llsl002.db&recNum=411 ...
You will note though that the act of 1806 only made them legal tender for a period of three years. There were several different acts over the years that extended the legal tender provisions but there were also periods where they were allowed to expire and foreign coins were not legal tender until new acts were passed. Also at times the coins specific coins made legal tender were changed. You will also notice that although the act of 1806 says that gold and silver foreign coins will pass as legal tender it also lists only specific coins and the rates at which they will pass.