Let's assume the P/E is a function of expectations for future sales and earnings. Walgreens has been opening about 400 stores a year for five or more years now. 400 new stores on a base of 4,000 produces some real future earnings benefits. when the store base is 7,000 the earnings benefit in dollars may be the same but in terms of percentage growth it's only half . A single strategy to open more stores as a source of increased profitability is going to continue to produce diminishing benefits. An expectation of slow earnings growth combined with a fairly low yield is going to produce a fairly low P/E.
You won't see a store in Japan for a long time. True, international opportunities were investigated years ago. After paying considerable fees to consultants the idea was scrapped. Walgreens opened a store in Hawaii a year ago and have yet to open their second. If they can't figure out Hawaii, they certainly aren't going to try a Japaneese venture.