Same reasons - unexpected changes, financial problem, etc. Since 1948, until Kasparov-Karpov, there was a match every 3 years except revenge matches.
Then, Karpov Kasparov is complicated. 84 match was stopped and replayed in 85 under new conditions (24 games maximum and no revenge since next match). Then 86 was a revenge match and 87 regular match in the end of the cycle 84-87. Next match was played in 3 years - 90.
Well there are many changes in Championship cycle last years, there were knockout, round robin both for title and Candidates. Probably because of changes made in 2013 (round robin instead of knockout for Candidates) it was decided to make two matches in a row. If you check you will be surprised how many changes were made into championship cycle
ok then if that's the reason you are giving, then explain why like for example the kasparov years, there were some years that are skipped...or even championships that are way before that?