Hayanaratna 1.148

Otherwise with lunar years, an adhimasa (extra lunar month) may come. Then the Sun equal to the birth Sun can sometimes come twice in one lunar year. The weekday and the rest of those two years should then differ. But they would not, since the years gone are the same. In the same way, take the year after a year with an adhimasa. There the Sun may equal the birth Sun only after the lunar year has passed. Then the weekday and the rest would be off by one year's weekday and the rest. So the Shaka should be taken to start at the start of the solar year.

Simple meaning: Why the solar year: with an extra lunar month, lunar counting can give the wrong years gone. So the Shaka starts with the solar year.