There are a multitude of reasons as to why state constitutions are lengthier than the U.S. constitution. As there are 50 states, the reasons will vary. The tl;dr is mostly that states have more Opportunity to add to their constitutions, whereas the national constitution has long and politically difficult processes only.

Reasons can include:

Direct Referendum: States such as Missouri and California have the option to change laws AND add state constitutional amendments every few years by a citizen initiative. The Federal government does not have this opportunity.

Easier Processes: State legislatures like Arkansas can add an amendment with only a Simple Majority, not even a 2/3 majority, which is what is required of Congress to pass an amendment which then needs 3/4 of the states to add the amendment to the national constitution.

Broad or Specific: The national constitution is also very vague or does not specify protocols or touch on more than the most necessary topics. States, on the other hand, will add long passages with drawn out and specific wording so that the constitution is as specific as possible, some times the passages of a state constitution will read like the legalese of state statute, but only placed in a higher level of authority because Opportunity is easier and can protect from a state supreme court from striking it down. Unless that state supreme court finds the amendment to be contradictory to current passages, such as the Colorado Initiative 54

https://ballotpedia.org/Colorado_Campaign_Contributions_from_Government_Contractors,Initiative_54(2008)

To reiterate, these are only a few of the reasons why states have longer constitutions that the nation. In fact, many states rewrote their constitutions entirely in the first half of the 20th century, an attempt to modernize and overhaul old documents, but gave an opportunity to ever professionalizing politicians to add on clauses during state constitutional conventions.

Source: Political Scientist with a focus on state governments, legislatures, and political movements.