The first two and a half months of 2021 haven't been particularly great for Amazon.com (AMZN -0.09%) shareholders. The share price is down about 5% for the year while the Nasdaq Composite index is up over 3% and the S&P 500 is up 5%.

While I'm extremely bullish on Amazon -- it's my largest holding and I bought more last month -- it's worth noting there are a few reasons the stock could continue to underperform this year. 

Here are three big ones.

A truck with the Amazon Prime logo painted on its trailer.

Image source: Amazon.com.

No more COVID-19 tailwind

Amazon was a big beneficiary of the shift to online shopping amid the COVID-19 pandemic. U.S. e-commerce sales grew more than 36% over the past three quarters of the year, according to the U.S. Census Bureau. 

North American e-commerce grew nearly 32% for the full year, according to estimates from eMarketer. And global e-commerce wasn't too far behind, with 28% growth.

But Amazon will move beyond that tailwind this spring, and as brick-and-mortar stores reopen, shoppers may shift some of their purchases back to in-store transactions. eMarketer expects worldwide e-commerce sales growth to slow to 14% this year, and U.S. e-commerce may grow only 6%, according to its analysts' estimates. 

A stark slowdown in revenue growth could scare off a lot of investors. Any top-line miss versus Wall Street's expectations could easily send shares lower.

Amazon's going to spend a lot more on logistics this year

2021 will be a transformative year for Amazon Logistics. The company's airhub at Cincinnati/Northern Kentucky International Airport will start operations later this year, with capacity for 200 flights per day. 

What's more, Amazon is making deals for more planes, looking to own a greater piece of its logistics business. It bought 11 planes this year. It also now owns a minority stake in cargo airline partner Air Transport Services Group after exercising warrants it acquired in its previous deals with the company. It has similar positions with other cargo carriers it could exercise as well.

Amazon will spend a lot of money on logistics network capacity this year, and that ought to continue for several years to come. CFO Brian Olsavsky has warned investors that capital expenditures will move higher. "We do see continued expansion on capex, specifically in our transportation area. So that will be the start of probably a multiyear period where we're higher on capex for that," he said during Amazon's third-quarter earnings call last year.

Amazon has historically done well by investing in additional shipping and logistics capabilities. The more in-depth expansion to truly own and control the logistics network could transform into an even bigger opportunity that more efficient shipping for the retail business. It could become a true competitor for logistics services. 

But increased spending will put a short-term damper on profits and cash flow, which could give investors of the high-priced stock concern.

Looming regulatory risk

One factor that's affecting all big tech stocks right now is the risk of increased government regulations forcing them to dramatically change company policies and operations or divest assets.

As Amazon's business has evolved to offer more third-party seller services for small businesses and advertising, it faces increased scrutiny to ensure it's not creating an anticompetitive environment with its own retail operations and products.

That said, it's likely in Amazon's best interest to make its third-party merchant services, advertising products, and other services as attractive as possible. They offer better margin profiles than Amazon's core retail business, and they offer greater revenue growth potential.

So while new regulations, or just the threat of regulations, could hurt Amazon's share price in the short run, the risk that they'll negatively affect Amazon's business and profits appears minimal.

Still a long-term growth story

If Amazon stock continues to underperform the market in 2021, it could present another opportunity for investors to buy shares. The current pullback in share price made the stock very attractive, but it wouldn't be a surprise if there are additional opportunities to snap up shares later this year as well.