It's not though. Many people use different measures of time all over the world. People use different calendars, and different holiday schedules, for example. Some notable examples include the Hebrew calendar, the Islamic calendar, the Japanese calendar, and the traditional Chinese calendar. On top of those there are often religious observations which sub-divide the day differently than the "SI" day or calendar. For example, many Jewish and Islamic observances, such as the observance of refraining from work on Shabbat or of fasting during Ramadan, which do not line up with the 24 hour day and are instead aligned with local sunrise and/or sunset.

These systems all co-exist with the SI system of the 24 hour day et all in the same way that countries that use the metric system can still use units like stone for weight or knots for speed.

As to why everyone came to adopt a universal, standardized system of time (even if it's not the only system of time in use), mostly that came down to industrialization and especially long distance travel. I won't go into the differences between different kinds of cultural approaches to time, I'll simply point out that industrialization and especially commercial mass transit (buses, railroads, etc.) all running on a fixed schedule required people locally to have an agreed upon synchronized shared time of day. However, even through much of the 19th century you would find that the local time of day would be different from one city to the next. Tables of time differences were often published in newspapers so that people who were traveling by rail between different cities would be able to plan appropriately. This, of course, became a logistical nightmare, and in many countries many railroads started keeping their own consistent time, called "railway time", which might differ from the local customary time in a particular city.

Meanwhile, at sea you also have another phenomenon: the longitude problem. Finding your latitude to some degree of accuracy at sea is fairly straightforward because if you know the day of the year and you can determine the angle the Sun sits at above the horizon at its highest point during the day (local noon) you can figure out your latitude. However, the same is not true of longitude. Which means, it could be easy to be off by many miles of your position over a long ocean going voyage, and that could be very dangerous. You could miss your destination entirely if it is a small island, or you could erroneously think you are farther than you actually are from a dangerous rocky outcrop or reef that could sink your vessel. There were many different attempts to solve the longitude problem but the one that finally became the most reliable and standardized was using time and the invention of a device which could keep track of time well over long voyages: the chronometer. If you know the exact current time at some fixed (other) location on Earth then you can determine your local longitude relative to that location by determining the local time (by measuring the moment when the Sun achieves its zenith, for example) and noting how that differs from the remote time. If you find that local noon is exactly one hour later at your current location than a known location then you know that you are exactly 1/24th around the circumference of the Earth to the West from that known location. Which means, if you can keep track of time to a precision of about 4 seconds or so you can keep track of your longitudinal position to an accuracy of a nautical mile (at the equator, more accurately closer to the poles).

The marine chronometer was essentially a very fancy escapement driven watch mechanism, which could carry a universal reference time across the world and keep synchronized with a drift of only a few seconds over a period of many months. The British developed this system and used the Greenwich Observatory near London as the geological and chronological reference point for this system, leading to the advent of "Greenwich Mean Time" or GMT.

In Britain railway time and GMT were synced up fairly early compared to the rest of the world, becoming standardized in the 1840s. In the rest of the world it took longer and occurred in fits and starts. Eventually the world ended up adopting Britain's standardization of both longitude (centered on Greenwich Observatory at 0 deg.) and time. Various conferences (such as the International Meridian Conference of 1884) occurred and legislation was passed. As countries adopted their own standardized time systems with rigid definitions of time zones and with everything in each time zone being synchronized (instead of, say, the local city clocks being different from the railroad clocks) they tended to do so within the context of GMT and "British" longitude.

Transcontinental railways and an increase in ocean travel made these sorts of standardizations more necessary but the real clinchers were the advent of the telegraph, telephone, and radio. With the whole world being connected at nearly the speed of light, it not only became more necessary to keep coordinated standard time between far distant regions, it also became far easier. Most countries adopted some system of standard time with fixed time zones (almost all of which were in one hour increment offsets from GMT) in the early 20th century, and by the 1930s almost every country on Earth was using such a system.