。
It was early spring, so there weren't many customers.
Literal
Spring-beginning is [because-から] customers [also-も] not-very were-not.
A short descriptive sentence. 春先 (はるさき) is 'early spring' — 春 (spring) + 先 (beginning, leading edge). Pairs with 夏先 (early summer), 秋先 (early autumn), 冬先 (early winter). Note this 先 means 'beginning, leading part,' not 'before' or 'tip' — a specific sense in seasonal compounds. あまり~ない ('not very') is the gradable-negation hedge for softening a negative. The 客も ('customers too') uses も implicitly to say 'as is typical for this time of year, customers were also scarce,' adding this absence to an implied context.