Why are write/wrote, ride/rode but strike/struck

The difference does not go back to Old English: in that language, the verbs ancestral to ride and strike both used the same vowels in the past tense:

• ride = Old English rīdan (inf.), rād (1s past), ridon (plr past), ġeriden (pp.)

• strike = Old English strīcan (inf.), strāc (1s past), stricon (plr past), ġestricen (pp.)

They are categorized as "class 1" strong verbs. Even though these kinds of verbs are now taught as "irregular", they did originally follow patterns.

The vowel changed in the past tense of the descendant of strīcan, but not of rīdan. Two broad and distinct categories of explanations for why sounds change in language are regular sound laws, and analogy. The change of vowel in the past tense of strike does not appear to follow a regular sound law: for comparison, the sequence āc yields the expected outcome of -oke in the noun spoke from Old English spāca. Rather, the reason for the u in struck seems to be analogy with how other verbs formed their past tense; the analogy seems to have been mainly based on the other verbs having similar consonant patterns, but possibly partly based on them having similar meanings.

A history of ablaut in the strong verbs from Caxton to the end of the Elizabethan period, by H.T. Price (1910), has some discussion of this verb's forms. Price says that stroke, the regular outcome of the Old English form, is common up until about 1600, and from then on tends to get replaced by strook and struck (page 23). Price's explanation is that struck was influenced by the past tense form stuck of the verb stick.

The Oxford English Dictionary entry for strike provides a record of forms that more or less agrees with Price's chronology, but does not discuss the irregular development.

The occurrence of u in stuck is itself hard to explain, as the forms of the verb "stick" have a complicated history.

There seem to be a small number of verbs ending in a velar consonant (/k/, /g/ or /ŋ/) which form past tenses with the vowel u (/ʌ/) in ways that don't go back to Old English. A handout on Anthony Kroch's website titled "The progress of /ʌ/" has a section "4. Early Modern English additions" which mentions the examples of strung, dug, hung in addition to stuck and struck, and in a later section mentions the development in modern English of "snuck" as a past tense of sneak.

"Survival of the Strongest", by Sherrylyn Branchaw (2010), suggests that innovative past tense forms with /ʌ/ are associated with verbs ending in a velar consonant due to the influence of verbs ending in -ng and -nk that were originally class 3 strong verbs, such as sting/stung or cling/clung (page 91, Studies in the History of the English Language V: Variation and Change in English Grammar and Lexicon: Contemporary Approaches, edited by Robert A. Cloutier, Anne Marie Hamilton-Brehm, and William A. Kretzschmar, Jr.). Branchaw also says that the s-initial consonant clusters shared between some of these verbs made it easier for them to influence each other's past tense forms.