The use of the present tense is in this passage is perfectly acceptable. Even though Alma is describing past events, the reason for why those events occurred remains true in present time and, more generally, throughout time.
Here in Alma 33:20, the 1840 edition changed the present-tense is to the past-tense was, which has been maintained throughout the RLDS textual tradition. This change to was could have been the result of Joseph Smith’s editing for the 1840 edition—or it may simply represent a typo on the part of the 1840 typesetter. Note that the larger narrative is all in the past tense: “a type was raised up … and many did look … but few understood … but there were many which were so hardened … therefore they perished ... because they did not believe” (Alma 33:19–20). Since the original present-tense reading works (“the reason … is because … ”), the critical text will maintain it.
Summary: Accept the original present-tense is in Alma 33:20 since the reason why something has occurred can be considered timeless (“now the reason that they would not look is because they did not believe that it would heal them”).