Cord-cutting isn't new, but its impact is being continually felt across the cable and satellite TV industries. A new report from the Leichtman Research Group found that in the first quarter of this year the largest pay-TV companies in the U.S. lost about 305,000 video subscribers.

The latest subscriber losses mark a continuation of consumers shunning traditional pay-TV services. In the first quarter of 2017, 515,000 pay-TV subscribers ditched cable and satellite packages.

Leichtman noted that the number of pay-TV subscribers peaked back in 2012 and that the top television providers have since lost 3.4 million subscribers -- and there's little to indicate that this trend will reverse. eMarketer estimates that the number of cord-cutters will increase to about 40.1 million in 2021.

That's bad news all around for cable and satellite providers, but it's becoming increasingly clear that Comcast (NASDAQ:CMCSA) could feel the brunt of this trend.

Why is Comcast vulnerable?

A research note published by UBS analyst John Hodulik at the end of March said that AT&T(NYSE:T) and Verizon would experience subscriber losses due to cord-cutting, but that Comcast would face the largest blow, as it could lose potentially 400,000 video subscribers this year.

So why is Comcast especially vulnerable? The company hasn't been as quick to shift toward over-the-top television streaming services.

Although Comcast does have its own internet-based television streaming service, called Xfinity Instant TV, which launched in late 2017, it has some fundamental limitations. The first being that the service is only available to Comcast's broadband customers, which severely limits the company's ability to extend its reach.

Second, and most importantly, Xfinity Instant only allows subscribers to watch live TV when they're connected to their home internet. If they go out of town or over to a friend's house, they can only access their DVR recordings, and the company's TV Go content. In contrast, similar services from other providers don't have the same restrictions.

Comcast's competitors offer streaming TV services to everyone -- even people who don't sign up for their internet services -- and their subscribers are growing. AT&T's DirecTV Now streaming service has about 1.5 million subscribers and is beginning to catch up with DISH Network's Sling TV's 2.3 million subscribers.

Those numbers are only the beginning, though, as streaming TV pressure on Comcast is only going to ramp up. AT&T is expected to expand further into the television streaming business through its recently announcement of AT&T Watch, which will start at an industry-low price of just $15 per month. AT&T is also going to give the service away for free to its wireless phone customers who have unlimited data plans.

Comcast isn't doomed, but it needs to change strategies

Here's where Comcast is right now: The company is hemorrhaging television customers due to cord-cutters. Its only streaming television service was late to the game, is only available to current Comcast internet customers, and doesn't offer the same live TV options as its competitors.

Some Comcast bulls will point out that the media giant jointly owns Hulu with Disney and Fox and that this partial ownership helps expose the company to the streaming TV market. But Hulu is more of a side hustle for these companies and only brought in $1 billion in revenue last year. When divided up among several companies, Hulu is hardly a drop in the bucket for these media powerhouses.

To fix its current problem, Comcast needs to open up Xfinity Instant to other customers -- outside of its current internet subscribers -- and remove its live TV restrictions. That may be hard to do as it likely negotiated its content programming deals to only include availability to existing broadband customers.

But if the company doesn't change its strategy soon, it'll miss out on the growing number of streaming TV subscribers. eMarketer expects the number of U.S. TV streaming customers to jump from 147.3 million in 2017 to 164.6 million in 2021. If Comcast continues to offer a restrictive service while its competitors provide flexible options that are available to anyone, it will not only suffer from traditional cable subscriber losses, but it will fail to win them back through new services as well.