The main reasons for this are that:
It is much easier to walk than Samaria: walking distance is 8 km, total descent is around 600m and the average walking time is about 2.5 hours. The path is generally easier underfoot.
This is the sort of walk that you could easily walk with children. It also would not need to take a whole long day.
The gorge of Imbros is easier to access: there is a good road (to Hora Sfakion and South Crete running along the upper part of the gorge) and no ferryboat trip is needed to get you back.
Despite being much smaller in size than Samaria there are some very spectacular sections, especially the several very narrow passages.
The gorge is open all year around. This makes it especially popular in spring time (April) when the gorge of Samaria is generally still closed.
For a number of people (especially Australians and New Zealanders and to a lesser extent Brits) walking through the gorge of Imbros is almost a pilgrimage. Many people have had parents or grand-parents who fought with the Allied troops in Crete and walked through the gorge in May 1941 in order to escape the island to Egypt.
The path into the gorge starts just below the village of Imbros. There are several cafés and tavernas (with ample space for parking) in Imbros and the routes to the entrance of the gorge are clearly marked. Depending on where you start from in the village it will take you between 3 and 10 minutes to get to a small house where you will need to purchase an entrance ticket (2 Euro in 2016).
From there the path descends gently into the gorge heading south. It is never steep and not particularly difficult (running shoes are good enough for this walk) but as everywhere in Crete the path is stony so you need to pay attention to where you put your feet.
Because the gorge is fairly narrow (under 2 meters wide at one point!) and has plenty of trees you will have shade most of the time.
The gorge is much smaller in height as well as length and less majestic than Samaria but this also means that at all times you are right next to the slopes and cliffs and have a much better opportunity to see plants and flowers at close range. And there are plenty of interesting flowers to see, especially in the spring.
After around 90 minutes walk from the gorge entrance booth (2 hours if you walk really slowly) you will pass an impressive stone arch on your right. This is almost the end of the route and if you are intending to walk back up to Imbros it is a good point to turn back. After this the gorge widens a little and after around 15 more minutes you get to the road and the village of Komitades where you'll find quite a few tavernas.
If you are on an organized bus tour this is where your bus will pick you up. If you need to go back to Imbros (or Hora Sfakion) ask in any of the tavernas. Plenty of the locals run unofficial taxi services (often in the back of a pickup which is quite an adventure) to cater for these needs.